Peer-Reviewed Articles

Articles dans des revues avec comité de lecture

2023

Supramolecular protection with a recyclable molecular container: an efficient strategy for the one-pot selective functionalization of polyfunctional substrates

The use of a supramolecular protecting molecular container, whose host-guest properties are under acid-base control, enables the functionalization with high chemo-, regio- and iteroselectivities of polyamines in a one-pot cyclic process.

Transmembrane Transport of Inorganic Phosphate by a Strapped Calix[4]pyrrole

Synthetic anion receptors are increasingly being explored for the transport of anions across lipid membranes because of their potential therapeutic applications. A considerable amount of research focuses on the transport of chloride, whereas the transmembrane transport of inorganic phosphate has not been reported to date, despite the biological relevance of this anion. Here we present a calix[4]pyrrole with a bisurea strap that functions as a receptor and transporter for H2PO4-, relying on the formation of eight hydrogen bonds and efficient encapsulation of the anion. Using a phosphate-sensitive lanthanide probe and 31P NMR spectroscopy, we demonstrate that this receptor can transport phosphate into vesicles by H2PO4-/Cl- antiport, H2PO4- uniport, and Cs+/H2PO4- symport mechanisms. This first example of inorganic phosphate transport by a neutral receptor opens perspectives for the future development of transporters for various biological phosphates.

Vanadium Catalyst in Micelles: Toward a Greener Aerobic Oxidative Cleavage of Vicinal Diols in Water

Aqueous micellar media offer an alternative to organic solvents for the development of safe and environmentally benign synthesis. A vanadium aminotriphenolate complex, known to be an effective catalyst for the aerobic oxidative C-C cleavage of vicinal diols in organic solvents, was incorporated in zwitterionic dodecyl phosphocholine (DPC) and in mixed TPGS-750-M:DPC micelles. It efficiently performed affording the corresponding carbonyl derivatives with no overoxidation, with catalyst loading down to 0.2 mol% and reaching TONs up to 500. The mixed micelles, which combine the enhanced stability conferred by zwitterionic DPC and the exceptional extraction properties of non-ionic TPGS-750-M, could be recycled and full conversion achieved upon reloading with catalyst and substrate. The study of the correlation between the reactivity of different substrates and their local concentration in the micellar core, derived from DOSY experiments, highlights that, while lipophilicity is important, the ease with which the substrate can access the catalyst, and therefore the location of the catalyst in the micellar phase, must also be considered when trying to rationalize reactivity in micelles.

2022

Specific Binding of Primary Ammoniums in Aqueous Media by Homooxacalixarenes Incorporated into Micelles

The development of artificial receptors for efficient recognition of analytes in water is a challenging task. Homooxacalix[3]arene-based receptor 1, which is selective toward primary ammoniums in organic solvents, was transferred into water following two different strategies: direct solubilization and micellar incorporation. Extensive 1H NMR studies showed that recognition of ammoniums is only observed in the case of micellar incorporation,highlighting the beneficial effect of the microenvironment of the micellar core. The selectivity of the system for primary ammoniums over secondary and tertiary ones was also maintained. The hydrophobic effect plays an important role in the recognition properties, which are counterion-dependent due to the energy penalty for the dissociation of certain ammonium salts in the apolar micellar core. This study shows that the straightforward self-assembly process used for the encapsulation of artificial receptors in micelles is an efficient strategy for developing water-soluble nanosized supramolecular recognition systems.

Calix[6]arenes with halogen bond donor groups as selective and efficient anion transporters

2021

A Water Molecule Triggers Guest Exchange at a Mono‐zinc Centre Confined in a Biomimetic Calixarene Pocket: a Model for Understanding Ligand Stability in Zn Proteins

In this study, the ligand exchange mechanism at a biomimetic ZnII centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene-based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand. These results advocate for an associative exchange mechanism involving the transient exo-coordination of a water molecule, giving rise to 5-coordinate ZnII intermediates, and inversion of the pyramid at the ZnII centre. Theoretical modelling by DFT confirmed that the associative mechanism is at stake. These results are particularly relevant in the context of the understanding of kinetic stability/lability in Zn proteins and highlight the key role that a single water molecule can play in catalysing ligand exchange and controlling the lability of ZnII in proteins.

2020

Specific Binding of Primary Ammonium Ions and Lysine-Containing Peptides in Protic Solvents by Hexahomotrioxacalix[3]arenes

The binding of ammonium ions by two homooxacalix[3]arene-based receptors was studied using NMR spectroscopy and in silico methods. Both receptors are shown to endocomplex, even in a protic environment, a large variety of primary ammonium ions, including biomolecules. The bindingmode is similar for all guests with the ammonium ion deeply inserted into the polyaromatic cavity and its NH3+ head nearly in the plane defined by the three oxygen atoms of the 18-crown-3 moiety, thus enabling it to establish three H-bonds with the ethereal macrocycle. The remarkable electronic, size, and shape complementarity between primary ammonium ions and the two cavitybased receptors leads to an unprecedented specificity for primary ammonium ions over secondary, tertiary, and quaternary ones. These binding properties were exploited for the selective liquid− liquid extraction of primary ammonium salts from water and for the selective recognition of lysine-containing peptides, opening new perspectives in the field of peptide sensing.

2019

Repositioning Chloride Transmembrane Transporters: Transport of Organic Ion Pairs

Given the biological importance of organic cations, the facilitated transport of organic ion pairs could find many applications. Calix[6]arene tris(thio)ureas, which possess a cavity that can accommodate primary ammonium ions, can not only act as carriers for Cl − /NO 3 − antiport but can also perform the cotransport of PrNH 3 Cl. Transport was monitored by fluorescence spectroscopy and the presence of the different species inside the vesicles was characterized by 1 H and 35 Cl NMR experiments involving shift reagents. The cotransport of PrNH 3 Cl was also observed by receptors deprived of a cavity, but the presence of the cavity conveys an advantage, as the cotransport by calix[6]arenes was observed to be more efficient than the Cl − /NO 3 − antiport, which is not the case with receptors without a cavity. The role played by the cavity was further highlighted by the disappearance of this advantage when using a bulky ammonium ion, which cannot be complexed within the cavity.

Fluorinated Bambusurils as Highly Effective and Selective Transmembrane Cl-/HCO3- Antiporters

The exchange of chloride and bicarbonate across lipid bilayers is an important biological process. Synthetic molecules can act as mobile carriers for these anions, although most show little selectivity. Here we report on three bambus[6]uril macrocycles functionalized with fluorinated benzyl groups, which are able to exchange Cl− and HCO3− efficiently. Remarkably, rates for Cl−/NO3− exchange are two orders of magnitude lower. The higher rates of Cl−/HCO3− transport can be explained by the ability of the bambusurils to complex Cl− and HCO3− simultaneously, facilitating their exchange at the bilayer interface. Furthermore, the exceptionally high affinity and selectivity of these systems for NO3− appear to contribute to the poor Cl−/NO3− exchange. This work not only demonstrates the importance of anion binding characteristics on anion transport but also the potential relevance of bambusurils for anion transport applications considering the high rate observed for Cl−/HCO3− exchange.

2018

Submerging a Biomimetic Metallo-Receptor in Water for Molecular Recognition: Micellar Incorporation or Water Solubilization? A Case Study

Molecular recognition in water is an important topic, but a challenging task due to the very competitive nature of the medium. The focus of this study is the comparison of two different strategies for the water solubilization of a biomimetic metallo-receptor based on a poly(imidazole) resorcinarene core. The first relies on a new synthetic path for the introduction of hydrophilic substituents on the receptor, at a remote distance from the coordination site. The second involves the incorporation of the organosoluble metallo-receptor into dodecylphosphocholine (DPC) micelles, which mimic the proteic surrounding of the active site of metallo-enzymes. The resorcinarene ligand can be transferred into water through both strategies, in which it binds ZnII over a wide pH window. Quite surprisingly, very similar metal ion affinities, pH responses, and recognition properties were observed with both strategies. The systems behave as remarkable receptors for small organic anions in water at near-physiological pH. These results show that, provided the biomimetic site is well structured and presents a recognition pocket, the micellar environment has very little impact on either metal ion binding or guest hosting. Hence, micellar incorporation represents an easy alternative to difficult synthetic work, even for the binding of charged species (metal cations or anions), which opens new perspectives for molecular recognition in water, whether for sensing, transport, or catalysis.

Efficient Vanadium-Catalyzed Aerobic C−C Bond Oxidative Cleavage of Vicinal Diols

The aerobic oxidative C−C bond cleavage of vicinal diols catalyzed by vanadium amino triphenolates is described. Our results show that C−C bond cleavage can be performed in different solvents, under an air or oxygen atmosphere, with a large variety of glycols (cyclic or linear, with aromatic or aliphatic substituents) affording the corresponding carbonyl derivatives with high chemoselectivity. Reactions can be performed with as little as 10 ppm of catalyst reaching TON up to 81,000 and TOFs of up to 4150 h−1. A reaction mechanism, rationalized by density functional theory calculations, is also proposed. (Figure presented.).

2016

Colorimetric and fluorescence “turn-on” recognition of fluoride by a maleonitrile-based uranyl salen-complex

The synthesis and characterization of two new uranyl-salen complexes, 1-2, based on a 1,2-diaminomaleonitrile unit, is described. Spectroscopic studies to evaluate their potential as colorimetric probes for fluoride detection in chloroform and dichloromethane were undertaken. Compound 2 exhibits a ‘turn-on' response characterized by a naked-eye colorimetric change for the selective recognition of fluoride in both solvents. DFT calculations show that the stabilization energy for the formation of the host:guest complex follows the trend F− > Cl− > Br− hence supporting the experimental data.

A selective calix[6]arene-based fluorescent chemosensor for phosphatidylcholine type lipids

The development of chemosensors that can selectively detect phosphatidylcholines (PCs) in biological samples is of medical relevance considering the importance of these phospholipids in cell growth and survival. Their selective sensing over phosphatidylethanolamines (PEs) is however a challenging task. We report here on the chemosensing capacities of calix[6]tris-pyrenylurea 1, which is able to selectively interact with phosphatidylcholine-type lipids in organic media. Host 1 also binds them in a biphasic chloroform/water solution, opening the way to the design of selective chemosensors for these lipids in biological media. The results obtained by NMR, fluorescence spectroscopy and modelling studies show that the selectivity is the result of the high degree of complementarity between the lipids' zwitterionic phosphatidylcholine headgroup and the receptor's H-bonding donor site and hydrophobic pocket. The mode of recognition is reminiscent of natural systems, such as human phosphatidylcholine transfer proteins (PC-TPs), validating the biomimetic approach adopted in our work.

Rapid and Selective Detection of Proteins by Dual Trapping Using Gold Nanoparticles Functionalized with Peptide Aptamers

A colorimetric platform for the fast, simple, and selective detection of proteins of medical interest is presented. Detection is based on the aggregation of two batches of peptide functionalized gold nanoparticles via the dual-trapping of the protein of interest. As proof of concept, we applied our platform to the detection of the oncoprotein Mdm2. The peptide aptamers used for the functionalization are based on the reported binding sequences of proteins p53 and p14 for the oncoprotein. Rapid aggregation, and a color change from red to purple, was observed upon addition of Mdm2 with concentrations as low as 20 nM. The selectivity of the system was demonstrated by the lack of response upon addition of bovine serum albumin (in large excess) or of a truncated version of Mdm2, which lacks one of the peptide binding sites. A linear response was observed between 30 and 50 nM of Mdm2. The platform reported here is flexible and can be adapted for the detection of other proteins when two binding peptide aptamers can be identified. Unlike current immunoassay methods, it is a one-step and rapid method with an easy readout signal and low production costs.

A comprehensive study to protein retention in hydrophobic interaction chromatography

The effect of different kosmotropic/chaotropic salt systems on retention characteristics of intact proteins has been examined in hydrophobic interaction chromatography (HIC). The performance was assessed using different column chemistries, i.e., polyalkylamide, alkylamine incorporating hydrophobic moieties, and a butyl chemistry. Selectivity in HIC is mainly governed by the salt concentration and by the molal surface tension increment of the salt. Typically, a linear relationship between the natural logarithm of the retention factor and the salt concentration is obtained. Using a 250 mm long column packed with 5 μm polyalkylamide functionalized silica particles and applying a 30 min linear salt gradient, a peak capacity of 78 was achieved, allowing the baseline separation of seven intact proteins. The hydrophobicity index appeared to be a good indicator to predict the elution order of intact proteins in HIC mode. Furthermore, the effect of adding additives in the mobile phase, such as calcium chloride (stabilizing the 3D conformation of α-lactalbumin) and isopropanol, on retention properties has been assessed. Results indicate that HIC retention is also governed by conformational in the proteins which affect the number of accessible hydrophobic moieties.

2015

Amino Acid Induced Fractal Aggregation of Gold Nanoparticles: Why and How

Gold colloids are the object of many studies as they are reported to have potential biological sensing, imaging and drug de-livery applications. In the presence of certain amino acids the aggregation of the gold nanoparticles into linear structures is observed, as highlighted by the appearance of a second plasmon band in the UV-Vis spectra of the colloid. The mechanism behind this phenomenon is still under debate. In order to help elucidate this issue, the interaction between gold colloids and different amino acids, modified amino acids and molecules mimicking their side-chain was monitored by UV-Vis absorp-tion, DLS and TEM. The results show that phenomenon can be rationalized in terms of the Diffusion Limited Colloid Ag-gregation (DLCA) model which gives rise to the fractal aggregation colloids. The global charge of the compound, which in-fluences the ionic strength of the solution, and the ease with which the compound can interact with the GNPs and affect their surface potential, are, the two parameters which control the DLCA regime. Calculations based on the Derjaguin, Lan-dau, Verwey and Overbeek (DLVO) theory confirm all the experimental observations.

Astrobiology and the Possibility of Life on Earth and Elsewhere...

Astrobiology is an interdisciplinary scientific field not only focused on the search of extraterrestrial life, but also on deciphering the key environmental parameters that have enabled the emergence of life on Earth. Understanding these physical and chemical parameters is fundamental knowledge necessary not only for discovering life or signs of life on other planets, but also for understanding our own terrestrial environment. Therefore, astrobiology pushes us to combine different perspectives such as the conditions on the primitive Earth, the physicochemical limits of life, exploration of habitable environments in the Solar System, and the search for signatures of life in exoplanets. Chemists, biologists, geologists, planetologists and astrophysicists are contributing extensively to this interdisciplinary research field.From 2011 to 2014, the European Space Agency (ESA) had the initiative to gather a Topical Team of interdisciplinary scientists focused on astrobiology to review the profound transformations in the field that have occurred since the beginning of the new century. The presentpaper is an interdisciplinary review of current research in astrobiology, covering the major advances and main outlooks in the field. The following subjects will be reviewed and most recent discoveries will be highlighted: the new understanding of planetary system formation including the specificity of the Earth among the diversity of planets, the origin of water on Earth and its unique combined properties among solvents for the emergence of life, the idea that the Earth could have been habitable during the Hadean Era, the inventory of endogenous and exogenous sources of organic matter and new concepts about how chemistry could evolve towards biological molecules and biological systems. In addition, many new findings show the remarkable potential life has for adaptation and survival in extreme environments.All those results from different fields of science are guiding our perspectives and strategies to look for life in other Solar System objects as well as beyond, in extrasolar worlds.

Primary Amine Recognition in Water by a Calix[6]aza-cryptand Incorporated in Dodecylphosphocholine Micelles

Water is a unique solvent and the design of selective artificial hosts that can efficiently work in an aqueous medium is a challenging task. It is known that the calix[6]tren zinc complex can recognize neutral guests in organic solvents. This complex was incorporated into dodecylphosphocholine micelles (DPC) and studied by NMR. The incorporated complex is able to extract selectively primary amines from the aqueous environment driven by an important hydrophobic effect which also affects the selectivity of the complex for these amines. This work shows how the incorporation of organo-soluble receptors in micelles can be an elegant and very efficient strategy to obtain water compatible nanosized supramolecular recognition devices which can be prepared via a straightforward self-assembly process.

Fluoride Binding in Water with the Use of Micellar Nanodevices Based on Salophen Complexes

The use of micelles to transpose lipophilic receptors, such as uranyl-salophen complexes, into an aqueous environment is a valuable and versatile tool. Receptor 1 incorporated into CTABr micelles forms a supramolecular system that exhibits excellent binding properties towards fluoride in water, despite the competition of the aqueous medium. To fully evaluate the potential of micellar nanodevices, we extended our previous study to other types of surfactants and to a uranyl-salophen receptor with a more extended aromatic surface. Paramagnetic relaxation enhancement experiments were used to obtain information on the location of the two receptors within the micelles and complementary information was obtained from dynamic light scattering experiments. With these data it is possible to account for the key factors necessary to obtain an efficient supramolecular device for anion binding in water.

2014

Fluorescent Chemosensors for Anions and Contact Ion Pairs with a Cavity-Based Selectivity

The association of a concave macrocyclic compound to one or multiple fluorophores is an appealing strategy for the design of chemosensors. Indeed, as with biological systems, a cavity-based selectivity can be expected with such fluorescent receptors. Examples of calix[6]arene-based systems using this strategy are rare in the literature, and to our knowledge, no examples of fluorescent receptors that can bind organic contact ion pairs have been reported. This report describes the straightforward synthesis of fluorescent calix[6]arene-based receptors 4a and 4b bearing three pyrenyl subunits and the study of their binding properties toward anions and ammonium salts using different spectroscopies. It was found that receptor 4a exhibits a remarkable selectivity for the sulfate anion in DMSO, enabling its selective sensing by fluorescence spectroscopy. In CDCl3, the receptor is able to bind ammonium ions efficiently only in association with the sulfate anion. Interestingly, this cooperative binding of ammonium sulfate salts was also evidenced in a protic environment. Finally, a cavity-based selectivity in terms of size and shape of the guest was observed with both receptors 4a and 4b, opening interesting perspectives on the elaboration of fluorescent cavity-based systems for the selective sensing of biologically relevant ammonium salts such as neurotransmitters.

2013

Paramagnetic Relaxation Enhancement Experiments: A Valuable Tool for the Characterization of Micellar Nanodevices

Micellar incorporation of hydrophobic molecular receptors is a promising strategy to obtain efficient nanodevices that work in water. In order to fully evaluate the potential of this approach, information on the localization and orientation of the receptor inside the micelle are necessary. Systematic studies undertaken on a uranyl−salophen receptor incorporated into CTABr and CTACl micelles show that nuclear magnetic resonance paramagnetic relaxation enhancement (NMR-PRE) experiments are particularly suitable to provide this type of information. The effect on the measurements of surfactant concentration, nature of the surfactant polar head, and ionic strength is also reported. Notably the normalization procedure applied to the obtained data can be considered of general application, thus enabling the comparison of information collected for different types of supramolecular micelle/receptor systems.

DNA-Promoted Auto-Assembly of Gold Nanoparticles: Effect of the DNA Sequence on the Stability of the Assemblies

The use of deoxyribonucleic acid (DNA) oligonucleotides has proven to be a powerful and versatile strategy to assemble nanomaterials into two (2D) and three-dimensional (3D) superlattices. With the aim of contributing to the elucidation of the factors that affect the stability of this type of superlattices, the assembly of gold nanoparticles grafted with different DNA oligonucleotides was characterized by UV-Vis absorption spectroscopy as a function of temperature. After establishing an appropriate methodology the effect of (i) the length of the grafted oligonucleotides; (ii) the length of their complementary parts and also of (iii) the simultaneous grafting of different oligonucleotides was investigated. Our results indicate that the electrostatic repulsion between the particles and the cooperativity of the assembly process play crucial roles in the stability of the assemblies while the grafting density of the oligonucleotide strands seems to have little influence.

Polyoxometalates as a Novel Class of Artificial Proteases: Selective Hydrolysis of Lysozyme under Physiological pH and Temperature Promoted by a Cerium(IV) Keggin-Type Polyoxometalate

Hen-egg-white lysozyme (HEWL) is specifically cleaved at the Trp28-Val29 and Asn44-Arg45 peptide bonds in the presence of a Keggin-type [Ce(α-PW11O39)2]10- polyoxometalate (POM; 1) at pH 7.4 and 37 °C. The reactivity of 1 towards a range of dipeptides was also examined and the calculated reaction rates were comparable to those observed for the hydrolysis of HEWL. Experiments with α-lactalbumin (α-LA), a protein that is structurally highly homologous to HEWL but has a different surface potential, showed no evidence of hydrolysis, which indicates the importance of electrostatic interactions between 1 and the protein surface for the hydrolytic reaction to occur. A combination of spectroscopic techniques was used to reveal the molecular interactions between HEWL and 1 that lead to hydrolysis. NMR spectroscopy titration experiments showed that on protein addition the intensity of the 31P NMR signal of 1 gradually decreased due to the formation of a large protein/polyoxometalate complex and completely disappeared when the HEWL/1 ratio reached 1:2. Circular dichroism (CD) measurements of HEWL indicate that addition of 1 results in a clear decrease in the signal at λ=208 nm, which is attributed to changes in the α-helical content of the protein. 15N-1H heteronuclear single quantum coherence (HSQC) NMR measurements of HEWL in the presence of 1 reveal that the interaction is mainly observed for residues that are located in close proximity to the first site in the α-helical part of the structure (Trp28-Val29). The less pronounced NMR spectroscopic shifts around the second cleavage site (Asn44-Arg45), which is found in the β-strand region of the protein, might be caused by weaker metal-directed binding, compared with strong POM-directed binding at the first site. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

UV-Vis and NMR study of the formation of gold nanoparticles by citrate reduction: Observation of gold-citrate aggregates

The citrate reduction of gold(III) in water is one of the most commonly used synthetic pathways for the preparation of gold colloids. In order to gain insight into the formation of gold nanoparticles (GNPs) using this method, the synthesis of GNPs was undertaken under different experimental conditions and monitored in operando by UV-Vis spectroscopy. These experiments highlight that citrate should be polydeprotonated and that Au(III) should not be polyhydroxylated in order to obtain GNPs with a narrow size distribution. Samples taken during the reaction were also characterized by Nuclear Magnetic Resonance Spectroscopy (NMR) to monitor the various reaction products as a function of time. Diffusion Ordered SpectroscopY (DOSY) experiments allowed us to identify slow diffusing citrate - Au(I) or Au(0) complexes which could play a role in the formation of GNPs. © 2013 Elsevier Inc.

2012

Monitoring Fluoride Binding in DMSO: Why is a Singular Binding Behavior Observed?

Fluoride recognition by molecular receptors is attracting much interest in the scientific community and a variety of approaches are used to achieve recognition with high affinity and selectivity. Most of the synthetic systems reported have been extensively studied in organic solvents by using tetrabutylammonium fluoride (TBAF) as the source of fluoride. In many cases, titration behaviors are observed that cannot be ascribed to a classical 1:1 binding isotherm. By using UV/Vis, 19F NMR, and 1H NMR spectroscopy we have been able to highlight that the equilibrium between the fluoride and the corresponding bihalide ion, HF2-, which is inevitably generated in situ due to trace amounts of water, can be at the origin of this singular behavior. Our results highlight that when undertaking titrations with fluoride in DMSO, the data can be affected by the fluoride-bihalide equilibrium and that the latter species can even be the dominating species at low TBAF concentrations.

2011

The search for a deterministic origin for the presence of non-racemic amino-acids in meteorites: a computational approach

Amino-acid enantiomeric excesses (ee's) have been detected in different types of carbonaceous chondrites, all in favor of the L enantiomer. In this article, we discuss possible deterministic causes to the presence of these amino-acid ee's in meteorites and evaluate in particular enantioselective photolysis by circularly polarized light (CPL). The electronic circular dichroism spectra of a set of amino- and hydroxy-acids, all detected in chondritic matter but some with ee's and others without ee's, were calculated and compared. The spectra were calculated for the most stable conformation(s) of the considered molecules using quantum mechanical methods (density functional theory). Our results suggest that CPL photolysis in the gas phase was perhaps not at the origin of the presence of ee's in meteorites and that the search for another, but still unknown, deterministic cause must be seriously undertaken.

Prebiotic chemistry : A fuzzy field

If prebiotic chemistry is defined as the study of the chemical steps, which lead to the first organisms, a clear-cut definition of “living organism” is needed. Unfortunately, no unambiguous and universally accepted definition exists for the concept “living”. Under these conditions, fuzzy logic is probably the methodological tool that can best be used to handle questions pertaining to “the origin of life”. A conventional scale must, however, be defined which interestingly enough, depends necessarily on our present-day scientific knowledge.

2010

Is it useful to have a clear-cut definition of life? On the use of fuzzy logic in prebiotic chemistry.

Many scientists, including one of the authors of the present paper, have devoted time to try to find a definition for life (Bersini and Reisse 2007). It is clear that a consensus will never be reached but, more importantly, it seems that the issue itself could be without major interest. It is indeed impossible to define a "natural" frontier between non-living and living systems and therefore also impossible to define dichotomic criteria which could be used in order to classify systems in one of these two classes (living or non-living). Fuzzy logic provides a natural way to deal with problems where class membership lacks sharply defined criteria. It also offers the possibility to avoid losing time with unnecessary controversies such as deciding whether a virus is, or is not, a living system.

2009

Comparison of the thermodynamics and base-pair dynamics of a full LNA:DNA duplex and of the isosequential DNA:DNA duplex

Locked nucleic acids (LNA), conformationally restricted nucleotide analogues, are known to enhance pairing stability and selectivity toward complementary strands. With the aim to contribute to a better understanding of the origin of these effects, the structure, thermal stability, hybridization thermodynamics, and base-pair dynamics of a full-LNA:DNA heteroduplex and of its isosequential DNA:DNA homoduplex were monitored and compared. CD measurements highlight differences in the duplex structures: the homoduplex and heteroduplex present B-type and A-type helical conformations, respectively. The pairing of the hybrid duplex is characterized, at all temperatures monitored (between 15 and 37 degrees C), by a larger stability constant but a less favorable enthalpic term. A major contribution to this thermodynamic profile emanates from the presence of a hairpin structure in the LNA single strand which contributes favorably to the entropy of interaction but leads to an enthalpy penalty upon duplex formation. The base-pair opening dynamics of both systems was monitored by NMR spectroscopy via imino protons exchange measurements. The measurements highlight that hybrid G-C base-pairs present a longer base-pair lifetime and higher stability than natural G-C base-pairs, but that an LNA substitution in an A-T base-pair does not have a favorable effect on the stability. The thermodynamic and dynamic data confirm a more favorable stacking of the bases in the hybrid duplex. This study emphasizes the complementarities between dynamic and thermodynamical studies for the elucidation of the relevant factors in binding events.

Probing polymer colloids by 129Xe NMR.

Model aqueous dispersions of polystyrene, poly(methyl methacrylate), poly(n-butyl acrylate) and a statistical copolymer poly(n-butyl acrylate-co-methyl methacrylate) were studied using xenon NMR spectroscopy. The (129)Xe NMR spectra of these various latexes reveal qualitative and quantitative differences in the number of peaks and in their line widths and chemical shifts. Above the glass transition temperature, exchange between xenon sorbed in the particle core and free xenon outside the particles is fast on the (129)Xe spectral time-scale and a single (129)Xe signal is observed. At temperatures below the glass transition temperature, the exchange between sorbed and free xenon is slow on the (129)Xe spectral time-scale and two (129)Xe NMR signals can be observed. If the signal of sorbed (129)Xe is observed, its chemical shift, line width and integral relative to the integral of free (129)Xe can be used for the characterization of the particle core. The line width of free (129)Xe provides the residence time of xenon outside the particles and can be used to determine the rate constant characterizing the kinetics of penetration of xenon in the particles. This rate constant emerges as promising parameter for the characterization of the polymer particle surface.

2008

Fluoride binding in water: a new environment for a known receptor.

(Figure Presented) Binding in a micelle: In the presence of CTABr micelles (see picture), the salophen-UO2 complex 1 binds fluoride in water with the highest affinity ever recorded for a neutral receptor (K≈ ca. 104M-1). The receptor's location and its orientation within the micellar system are determined by PRE and NOE NMR experiments. © 2008 Wiley-VCH Verlag GmbH& Co. KGaA.

Aromatic-carbohydrate interactions: an NMR and computational study of model systems.

The interactions of simple carbohydrates with aromatic moieties have been investigated experimentally by NMR spectroscopy. The analysis of the changes in the chemical shifts of the sugar proton signals induced upon addition of aromatic entities has been interpreted in terms of interaction geometries. Phenol and aromatic amino acids (phenylalanine, tyrosine, tryptophan) have been used. The observed sugar-aromatic interactions depend on the chemical nature of the sugar, and thus on the stereochemistries of the different carbon atoms, and also on the solvent. A preliminary study of the solvation state of a model monosaccharide (methyl beta-galactopyranoside) in aqueous solution, both alone and in the presence of benzene and phenol, has also been carried out by monitoring of intermolecular homonuclear solvent-sugar and aromatic-sugar NOEs. These experimental results have been compared with those obtained by density functional theory methods and molecular mechanics calculations.

Catalysis by mettalloenzymes and the origin of life

2007

Protonation linked equilibria and apparent affinity constants: the thermodynamic profile of the alpha-chymotrypsin-proflavin interaction

Protonation/deprotonation equilibria are frequently linked to binding processes involving proteins. The presence of these thermodynamically linked equilibria affects the observable thermodynamic parameters of the interaction (K(obs), DeltaH(obs)(0) ). In order to try and elucidate the energetic factors that govern these binding processes, a complete thermodynamic characterisation of each intrinsic equilibrium linked to the complexation event is needed and should furthermore be correlated to structural information. We present here a detailed study, using NMR and ITC, of the interaction between alpha-chymotrypsin and one of its competitive inhibitors, proflavin. By performing proflavin titrations of the enzyme, at different pH values, we were able to highlight by NMR the effect of the complexation of the inhibitor on the ionisable residues of the catalytic triad of the enzyme. Using ITC we determined the intrinsic thermodynamic parameters of the different equilibria linked to the binding process. The possible driving forces of the interaction between alpha-chymotrypsin and proflavin are discussed in the light of the experimental data and on the basis of a model of the complex. This study emphasises the complementarities between ITC and NMR for the study of binding processes involving protonation/deprotonation equilibria.

Developments in the characterisation of the catalytic triad of alpha-chymotrypsin: Effect of the protonation state of Asp102 on the 1H NMR signals of His57

Protonated or not? 1H NMR spectra of α-chymotrypsin were recorded as a function of pH (from top to bottom pH 8.6, 5.1 and 4.2). Slow exchange is observed for the NH protons of His57 between two environments due to different protonation states of Asp102. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

2006

Novel method for the measurement of xenon gas solubility using 129Xe NMR spectroscopy.

A novel method is presented for determining xenon partitioning between a gas phase and a liquid phase. An experimental setup which permits the simultaneous measurement of the 129Xe chemical shift in both the gas and the liquid phases, that is, under the same experimental conditions, has been designed. Xenon solubility is obtained via 129Xe chemical shift measurements in the gas phase. The method was validated against xenon solubility data from the literature; in general, the agreement is found to be within 3%. The solubility of xenon in three solvents for which data have not been previously reported (acetone, acetonitrile, and 1,1,2,2-tetrachloroethane) was determined using this novel method. 129Xe chemical shifts for dissolved xenon are also reported; it is found that xenon-xenon interactions may play a significant role in the liquid phase even at low equilibrium xenon pressures.

Structural characterization of the papaya cysteine proteinases at low pH.

Current control of gastrointestinal nematodes relies primarily on the use of synthetic drugs and encounters serious problems of resistance. Oral administration of plant cysteine proteinases, known to be capable of damaging nematode cuticles, has recently been recommended to overcome these problems. This prompted us to examine if plant cysteine proteinases like the four papaya proteinases papain, caricain, chymopapain, and glycine endopeptidase that have been investigated here can survive acidic pH conditions and pepsin degradation. The four papaya proteinases have been found to undergo, at low pH, a conformational transition that instantaneously converts their native forms into molten globules that are quite unstable and rapidly degraded by pepsin. As shown by activity measurements, the denatured state of these proteinases which finally results from acid treatment is completely irreversible. It is concluded that cysteine proteinases from plant origin may require to be protected against both acid denaturation and proteolysis to be effective in the gut after oral administration.

Oligonucléotides chimiquement modifiés : outils diagnostiques et agents thérapeutiques prometteurs. Méthodes permettant d'étudier leur stabilité et sélectivité d'appariement

Do Serine Octamers Exist in Solution? Relevance of this Question in the Context of the Origin of Homochirality on Earth

Recent MS studies have suggested that serine clusters could have played a role in the origin of homochirality on Earth. Aqueous serine solutions have been probed in order to see if serine clusters, such as those observed by MS, are present in solution. IR measurements as well as NMR chemical shift and diffusion coefficient measurements, as a function of pH and serine concentration, suggest that these clusters do not exist in solution.

2005

Utilisation de la Calorimétrie à Titrage Isotherme pour l'étude des interactions entre (bio)molécules

Xenon NMR as a Probe for Microporous and Mesoporous Solids, Polymers, Liquid Crystals, Solutions, Flames, Proteins, Imaging

Cet article, coordonné par Jacques Fraissard, donne quelques exemples d'applications de la résonance magnétique nucléaire (RMN) du xénon utilisé comme sonde à l'étude de divers systèmes chimiques. Citons, de façon non exhaustive : la détermination de la porosité des solides micro- et mésoporeux, l'évaluation de la concentration et des dimensions des domaines amorphes dans les polymères solides, la caractérisation des cristaux liquides, l'étude des processus de combustion à haute température, la détermination de la structure et de la dynamique des systèmes organiques et de protéines en solution, l'estimation du débit sanguin dans le cerveau.

2004

Stereochemical studies by molecular palpation

The conformational equilibrium of cyclohexanol was investigated by 129X NMR spectroscopy. While the classical NMR approach focuses on a carbon or proton atom belonging to the molecule under investigation, in our 129Xe NMR methodology we use the xenon atom as an external spy. Xenon is able to monitor the interconversion between the: axial and the equatorial isomers of cyclohexanol. The conformational equilibrium constant was estimated and is in excellent agreement with the values obtained by 1H and 13C NMR. ©2004 John Wiley & Sons, Ltd.

2003

The Potential of the Xenon "Spin-Spy" Methodology for the Study of Configurational Equilibria in Solution

Xenon, a gas of rare ability: The mutarotation of D-glucose (see structure) was monitored using 129Xe NMR spectroscopy. This is the first example of the use of xenon NMR spectroscopy to probe a configurational equilibrium in solution of a system which does not complex xenon.

Comparison of the NMR enantiodifferentiation of a chiral ruthenium(II) complex of C2 symmetry using the TRISPHAT anion and a lanthanide shift reagent

The tris[tetrachlorobenzenediolato]phosphate(v) anion (TRISPHAT) is known to be an efficient NMR chiral shift agent for various chiral cationic species. Here we compare the efficiency of TRISPHAT and of a chiral lanthanide shift reagent for the determination of the enantiomeric purity of the chiral building block [Ru(phen)[2]PY[2]][2][+] which possesses C[2] symmetry. We also discuss our results in terms of the geometry of interaction between the Ru(II) complex and the TRISPHAT anion.

2002

The potential of 129Xe NMR relaxation measurements for the study of heme proteins.

Monoatomic xenon can be complexed in the proximal cavity of metmyoglobin. This study highlights the potential of 129Xe relaxation rate measurements to obtain information on the xenon-metmyoglobin interaction.

On the Origin of Homochirality on Earth

Origin of life: the role of water in the transition from non-living to living matter

2001

Probing proteins in solution by 129Xe NMR spectroscopy

The interaction of xenon with different proteins in aqueous solution is investigated by (129)Xe NMR spectroscopy. Chemical shifts are measured in horse metmyoglobin, hen egg white lysozyme, and horse cytochrome c solutions as a function of xenon concentration. In these systems, xenon is in fast exchange between all possible environments. The results suggest that nonspecific interactions exist between xenon and the protein exteriors and the data are analyzed in term of parameters which characterize the protein surfaces. The experimental data for horse metmyoglobin are interpreted using a model in which xenon forms a 1:1 complex with the protein and the chemical shift of the complexed xenon is reported (Locci et al., Keystone Symposia "Frontiers of NMR in Molecular Biology VI", Jan. 9--15, 1999, Breckenridge, CO, Abstract E216, p. 53; Locci et al., XeMAT 2000 "Optical Polarization and Xenon NMR of Materials", June 28--30, 2000, Sestri Levante, Italy, p. 46).

NMR Investigation of the complexation of neutral guests by cucurbituril

The effect of acidity on the stability of the complexes of neutral guests with cucurbituril (CB), a synthetic receptor, in aqueous Na2SO4 solutions is investigated by 129Xe, 19F and 1H NMR spectroscopy. The apparent affinity constants of THF and xenon for CB were measured and found to be independent of the pH of the solution. It was also observed that small acids, such as TFA, in their undissociated form can be encapsulated inside the CB cavity. The apparent association constants of THF, Xe and TFA for CB are estimated to be around 1700 M-1, 210 M-1 and 11 M-1, respectively.

NMR study of the reversible complexation of xenon by cucurbituril

The interaction of xenon with cucurbituril, a synthetic receptor, in aqueous solutions is investigated by 129Xe and 1H NMR spectroscopy. Xenon is reversibly trapped into the cavity to form a 1 1 host-guest complex. The exchange between the free and the complexed xenon is slow on the 129Xe NMR chemical shift time scale but fast on the 1H NMR chemical shift time scale. The apparent association constant of xenon for cucurbituril can be obtained from analysis of the 1H spectra and is estimated to be around 200 M-1 at 298 K. Interestingly, even though no H-bonding or electrostatic interactions play a role in the stabilisation of the complex, its stability is comparable to the stability of cucurbituril complexes with certain aryl- and alkyl-ammonium ions.

Molecular polarization and molecular chiralization: The first example of a chiralized xenon atom

In this article we focus on the interaction between a chiral molecule and a single achiral molecule or an ensemble of achiral molecules. The desymmetrization of the achiral molecules resulting from this interaction is described as ldquochiralization.rdquo By analogy with electric polarization, we factorize chiralization into three factors, i.e., orientation, atomic, and electronic terms. Chiralization depends on the dipolar polarizability of the chiralized molecule but also on polarizabilities of higher order. The experimental part of this work is devoted to the electronic chiralization of a xenon atom and its observation by 129Xe NMR spectroscopy.

2000

About a Double-Body Immersion Horn System to be Used for Quantitative Sonochemical Studies

The majority of quantitative sonochemical studies in the 20-100-kHz frequency range are performed by using an immersion horn system. The new system described in this letter consists of a double immersion horn acted on by a single pair of piezoelectric ceramics. This instrument is well adapted to the quantitative measure of effects, i.e., variations in the rate constant of a specific reaction associated to the change of an experimental parameter. The gas effect, obtained by comparing the rate of a reaction under air and under argon, illustrates the efficiency of the system.

Can Monoatomic Xenon Become Chiral ?

A chiral host, cryptophane-A (1), makes even a monoatomic noble gas chiral. The interaction of xenon and 1 was monitored by 129Xe NMR and in the presence of a chiral chemical shift reagent.

A Brief Survey of Organic Matter in the Universe

Study by 23Na-NMR, 1H-NMR, and Ultraviolet Spectroscopy of the Thermal Stability of an 11-basepair Oligonucleotide

1998

129Xe and 1H NMR Study of the Reversible Trapping of Xenon by Cryptophane-A in Organic Solution

The interaction of xenon with cryptophane-A in 1,1,2,2-tetrachloroethane-d2 is investigated by 129Xe and 1H NMR spectroscopy. Xenon is reversibly trapped into the cavity of this host to form a 1 to 1 host−guest complex with an apparent association constant K of the order of at least 3 × 103 M-1 at 278 K. The exchange between the free and bound xenon is slow on the 129Xe NMR time scale, and the bound xenon resonance is shifted by approximately 160 ppm to lower frequencies with respect to the free xenon resonance. The xenon complex is at least 4 and 20 times more stable, respectively, than the corresponding chloroform and methane complexes under the same conditions. The stability of this xenon complex appears to be much greater than that of the previously described xenon complex of α-cyclodextrin in water. This is probably due to the combination of three favorable effects: (i) good size matching between the guest and the cryptophane cavity in its most relaxed conformation, resulting in the optimization of the London forces between the highly polarizable guest and the electron rich aromatic rings of the host (enthalpic stabilization); (ii) no rotational or vibrational entropy loss of the monatomic guest in the cryptophane cavity; and (iii) no (or little) entropy loss due to reduction of the conformational freedom of the host. Analysis of the line widths of the signals corresponding to the free and bound xenon as a function of the relative xenon/cryptophane ratio suggests that the incoming xenon atom must displace the departing one to enter the cryptophane cavity, and that the empty cryptophane is not involved in the complexation equilibrium.

Pulsed Sonochemistry

Very few papers are devoted to the study of the chemical effects of pulsed ultrasound in the low-frequency range. The present work consists of a systematic experimental study of the effects of pulsed ultrasound in the 20 kHz range using an immersed titanium horn. The light scattered by the bubble cloud, the acoustic pressure, and the sonochemical activity were measured. The sonochemical activity was studied by measuring the light emitted by a fast chemiluminescent reaction (oxidation of luminol). The chemiluminescence behavior observed at 20 kHz was compared with the behavior observed at 1.7 MHz. The chemiluminescence takes time to install when sonication starts and, at 20 kHz, the luminescence intensity decreases monoexponentially when sonication stops. Interestingly, at 1.7 MHz, the luminescence intensity decreases biexponentially with an important fast component. The interpretation of these various behaviors requires that the acoustical characteristics of the ultrasound generator and of the vessel, and also the properties of the bubble field, be considered.

1997

Group Contribution Analysis of Xenon NMR Solvent Shifts

129Xe gas-to-solution NMR chemical shifts for xenon dissolved in pure n-alkanes, n-alkyl alcohols, n-alkyl carboxylic acids, di-n-alkyl ketones, and cycloalkanes and in solutions of lauric acid in n-heptane are reported. The medium effect corrected for solvent density is found to be linearly dependent on the number of carbon atoms except for the shortest members of the series of linear solvents. The same slope is observed for all the linear solvents; the slope for the cycloalkanes is significantly different. These results are interpreted on the basis of a group contribution analysis. The relative contribution of methyl and methylene groups in linear solvents is found to be in very good agreement with the relative XeCH3 and XeCH2 dispersive interaction energies. The 129Xe chemical shifts for solutions of lauric acid in n-heptane calculated from the group contributions are in excellent agreement with the experimental values. The deshielding effect of the methylene group in cycloalkanes and the 129Xe chemical shift measured in the shortest members of the linear series of solvent are discussed in terms of intermolecular shielding functions and distributions of groups in the solvation shell of the Xe atom.

1995

Probing molecular cavities in alpha-cyclodextrin solutions by Xenon NMR

[129]Xe NMR has been used to investigate intermolecular interactions between xenon and α-cyclodextrin in H[2]O and in DMSO. Nuclear Overhauser enhancement effects confirmed the formation of a xenon-cyclodextrin complex. Chemical shifts were measured in solutions containing different concentrations of α-cyclodextrin and in solutions containing linear oligosaccharides. The experimental data are interpreted on the basis of a three-site model for xenon. The binding characteristics of the xenon-cyclodextrin system, including the [129]Xe NMR chemical shift in the cavity and the equilibrium constants, were determined and are discussed.

1994

Measurement of the individual pKa values of acidic residues of hen and turkey lysozymes by two-dimensional 1H NMR.

The pH dependence of the two-dimensional 1H nuclear magnetic resonance spectra of hen and turkey egg-white lysozymes has been recorded over the pH range 1-7. By monitoring the chemical shifts of the resonances of the various protons of ionizable residues, individual pKa values for the acidic residues have been determined for both proteins. The pKa values are displaced, with the exception of those of the residues in the active site cleft, by an average of 1 unit to low pH compared to model compounds.

The importance of quadrupolar interactions in molecular recognition processes involving a phenyl group

1993

1H-NMR analysis of turkey egg-white lysozyme and comparison with hen egg-white lysozyme.

The complete main chain and approximately 75% of the side chain 1H-NMR assignments of the 129-residue protein, turkey egg-white lysozyme, are presented. NOE data, hydrogen-exchange rates, chemical shifts and coupling constants are reported and are indicative of a structure in solution that is essentially identical to that of the homologous hen egg-white lysozyme. The NH-alpha CH coupling constants of turkey lysozyme are compared to torsion-angle data from three crystal structures of the protein and the results are interpreted in terms of crystal-structure resolution and refinement.

A method for the estimation of χ1 torsion angles in proteins

Summary A method for estimating αCH-βCH coupling constants from the shape and fine structure of NH-αCH fingerprint-region cross peaks of COSY spectra is presented. Spectral simulations have been used to analyse the effect of variations in 3JNH-αCH, 3JαCH-βCH, linewidths and digital resolution on the appearance of NH-αCH COSY cross peaks. On the basis of these simulations a set of rules for broadly categorising experimental NH-αCH cross peaks according to αCH-βCH coupling constants has been devised. The method has been applied to the analysis of NH-αCH cross peaks of hen lysozyme. The results are compared to previous measurements of αCH-βCH coupling constants using E.COSY techniques.

1987

Topsentins, new toxic bis-indole alkaloids from the marine sponge Topsentia genitrix

Three new bis-indole alkaloids, topsentin-A (1), -B 1 (2), and -B2 (3) have been isolated from the Mediterranean sponge Topsentia genitrix and their structure determined by spectroscopic methods. These compounds are weakly toxic for fish and for dissociated cells of the freshwater sponge Ephydatiafluviatilis and thus might be partially responsible for the chemical defense of the sponge.

Articles dans des revues avec comité de lecture

2023

Development of a Peptide-based Lateral Flow Assay for the Detection of the Cancer Biomarker Mdm2

This study introduces peptide aptamers as a promising alternative to conventional antibodies for use as recognition units in lateral flow assays (LFAs). Two distinct strategies for immobilizing peptides onto nitrocellulose (NC) membranes were investigated: the first involved covalent coupling to bovine serum albumin (BSA) using EDC/sulfo-NHS chemistry, while the second utilized the well-known biotin/streptavidin complexation method. The former strategy only requires the addition of a lysine in the peptide sequence, while the latter exploits a commercially widely available chemical modification of these biomolecules. Both methods proved successful in immobilizing the capture probes onto the surface. Furthermore, we employed silver nanoplates functionalized with calixarenes as colorimetric reporters, that we have shown recently to exhibit excellent characteristics for LFAs, including high absorption coefficient, excellent stability, and strong contrast with the NC membrane. The peptide-based assay detected Mdm2, a well-established cancer biomarker, in low nanomolar range even in complex matrixes such as cell lysates. Notably, the utilization of peptide aptamers demonstrated superior performance and extended shelf life compared to polyclonal antibodies, underlining their potential as recognition units in LFAs. In addition to their ease of handling, peptide aptamers utilization also offers the prospect of substantial cost reductions compared to conventional antibody-based LFAs. This comprehensive approach enhances the utility of LFAs for the sensitive and costeffective detection of target proteins using peptide aptamers as recognition units both on the colorimetric reporter and the membrane, opening doors to broader applications in biomarker analysis and diagnostic assays.

Ultra-stable Silver Nanotriangles: Efficient and Versatile Colorimetric Reporters for Dipstick Assays

Noble metal anisotropic nanostructures, such as silver nanoplates (AgNPls), are interesting because they possess enhanced plasmonic properties compared to their spherical counterparts: increased extinction coefficient and tunable maximum of absorption wavelength. However, their use for biosensing application is limited as these structures are intrinsically unstable and, to maintain the anisotropic structure, a coating protecting the metallic surface is required. In this work, we report on the capacity of a thin but robust coating based on calixarene-diazonium salts to maintain the structure anisotropy of silver nanoplates in conditions in which traditionally used coatings fail. We synthesized AgNPls of various sizes and coated them with two different calixarenes, differing by the functional groups attached to their small rim. After characterization of the efficiency of the ligand exchange process between the initial citrate anions and the calixarenes, the chemical and colloidal stabilities of the resulting calixarene-coated AgNPls were compared to citrate-capped AgNPls. A radical improvement of the lifetime of the material from 1 day for AgNPls coated with citrate to more than 900 days for calixarene-coated AgNPls, as well as the stability in acidic conditions, phosphate saline buffer (PBS) or biofluid, were observed. Benefiting from this exceptional robustness, calixarene-coated AgNPls were exploited to design dipstick assays. Rabbit immunoglobulin G (IgG) detection was developed first as proof-of-concept. The optimal system was then used for the detection of Anti-SARS-CoV-2 IgG. In both cases, a picomolar limit of detection (LOD) was achieved as well as the detection in 100% of pooled human plasma. This sensitivity competes with that of ELISA and is better than the one previously obtained with gold or even silver nanospheres for the same target and in similar conditions. Finally, the wide range of colors provided by the AgNPls allowed the design of a multicolor multiplex assay for the simultaneous detection of multiple analytes.

Empirical Optimization of Peptide Sequence and Nanoparticle Colloidal Stability: The Impact of Surface Ligands and Implications for Colorimetric Sensing

Surface ligands play a critical role in controlling and defining the properties of colloidal nanocrystals. These aspects have been exploited to design nanoparticle aggregation-based colorimetric sensors. Here, we coated 13-nm gold nanoparticles (AuNPs) with a large library of ligands (e.g., from labile monodentate monomers to multicoordinating macromolecules) and evaluated their aggregation propensity in the presence of three peptides containing charged, thiolate, or aromatic amino acids. Our results show that AuNPs coated with the polyphenols and sulfonated phosphine ligands were good choices for electrostatic-based aggregation. AuNPs capped with citrate and labile-binding polymers worked well for dithiol-bridging and π-π stacking-induced aggregation. In the example of electrostatic-based assays, we stress that good sensing performance requires aggregating peptides of low charge valence paired with charged NPs with weak stability and vice versa. We then present a modular peptide containing versatile aggregating residues to agglomerate a variety of ligated AuNPs for colorimetric detection of the coronavirus main protease. Enzymatic cleavage liberates the peptide segment, which in turn triggers NP agglomeration and thus rapid color changes in <10 min. The protease detection limit is 2.5 nM.

A Fluoride-Induced Aggregation Test to Quickly Assess the Efficiency of Ligand Exchange Procedures from Citrate Capped AuNPs

2022

Atomic Force Manipulation of Single Magnetic Nanoparticles for Spin-Based Electronics

Magnetic nanoparticles (MNPs) are instrumental for fabrication of tailored nanomagnetic structures, especially where top-down lithographic patterning is not feasible. Here, we demonstrate precise and controllable manipulation of individual magnetite MNPs using the tip of an atomic force microscope. We verify our approach by placing a single MNP with a diameter of 50 nm on top of a 100 nm Hall bar fabricated in a quasi-two-dimensional electron gas (q2DEG) at the oxide interface between LaAlO3 and SrTiO3 (LAO/STO). A hysteresis loop due to the magnetic hysteresis properties of the magnetite MNPs was observed in the Hall resistance. Further, the effective coercivity of the Hall resistance hysteresis loop could be changed upon field cooling at different angles of the cooling field with respect to the measuring field. The effect is associated with the alignment of the MNP magnetic moment along the easy axis closest to the external field direction across the Verwey transition in magnetite. Our results can facilitate experimental realization of magnetic proximity devices using single MNPs and two-dimensional materials for spin-based nanoelectronics.

Bifunctional Calix[4]arene-Coated Gold Nanoparticles for Orthogonal Conjugation

Magnetic Self-Healing Composites: Synthesis and Applications

Magnetic composites and self-healing materials have been drawing much attention in their respective fields of application. Magnetic fillers enable changes in the material properties of objects, in the shapes and structures of objects, and ultimately in the motion and actuation of objects in response to the application of an external field. Self-healing materials possess the ability to repair incurred damage and consequently recover the functional properties during healing. The combination of these two unique features results in important advances in both fields. First, the self-healing ability enables the recovery of the magnetic properties of magnetic composites and structures to extend their service lifetimes in applications such as robotics and biomedicine. Second, magnetic (nano)particles offer many opportunities to improve the healing performance of the resulting self-healing magnetic composites. Magnetic fillers are used for the remote activation of thermal healing through inductive heating and for the closure of large damage by applying an alternating or constant external magnetic field, respectively. Furthermore, hard magnetic particles can be used to permanently magnetize self-healing composites to autonomously re-join severed parts. This paper reviews the synthesis, processing and manufacturing of magnetic self-healing composites for applications in health, robotic actuation, flexible electronics, and many more.

Ultrastable Silver Nanoparticles for Rapid Serology Detection of Anti-SARS-CoV-2 Immunoglobulins G

Dipstick assays using silver nanoparticles (AgNPs) stabilized by a thin calix[4]arene-based coating were developed and used for the detection of Anti-SARS-CoV-2 IgG in clinical samples. The calixarene-based coating enabled the covalent bioconjugation of the SARS-CoV-2 Spike Protein via the classical EDC/sulfo-NHS procedure. It further conferred remarkable stability to the resulting bioconjugated AgNPs, as no degradation was observed over several months. In comparison with lateral-flow immunoassays (LFIAs) based on classical gold nanoparticles, our AgNP-based system constitutes a clear step forward, as the limit of detection for Anti-SARS-CoV-2 IgG was reduced by 1 order of magnitude and similar signals were observed with 10 times fewer particles. In real clinical samples, the AgNP-based dipstick assays showed impressive results: 100% specificity was observed for negative samples, while a sensitivity of 73% was determined for positive samples. These values match the typical sensitivities obtained for reported LFIAs based on gold nanoparticles. These results (i) represent one of the first examples of the use of AgNP-based dipstick assays in the case of real clinical samples, (ii) demonstrate that ultrastable calixarene-coated AgNPs could advantageously replace AuNPs in LFIAs, and thus (iii) open new perspectives in the field of rapid diagnostic tests.

Highly stable silver nanohybrid electrocatalysts for the oxygen reduction reaction

Silver nanoparticles (AgNPs) were deliberately functionalized via aryl diazonium chemistry with a monolayer of calix[4]arenes. The resulting nanohybrids show high efficiency and high selectivity toward the ORR in alkaline media along with an exceptional durability and a high methanol tolerance.

2021

Peptide‐Conjugated Silver Nanoparticles for the Colorimetric Detection of the Oncoprotein Mdm2 in Human Serum

The development of efficient, reliable, and easy-to-use biosensors allowing early cancer diagnosis is of paramount importance for patients. Herein, we report a biosensor based on silver nanoparticles functionalized by peptide aptamers for the detection of a cancer biomarker, i.e. the Mdm2 protein. Silver nanoparticles (AgNPs) were produced and stabilized with a thin PEGylated-calix[4]arene layer that allows (i) the steric stabilization of the AgNPs and (ii) the covalent conjugation of the peptide aptamers via the formation of an amide bond. These peptide-conjugated AgNPs were then used to detect Mdm2 via a dual trapping strategy that was previously reported with gold nanoparticles (AuNPs). Our results showed that replacing AuNPs by AgNPs allows to improve the detection limit by nearly one order of magnitude, down to 5 nM, while the high selectivity of the system and the stability of the particles provided by the calixarene coating allow the detection of Mdm2 in human serum.

Synthesis of Ultrastable and Bioconjugable Ag, Au, and Bimetallic Ag_Au Nanoparticles Coated with Calix[4]arenes

Compared to gold nanoparticles, silver nanoparticles are largely underexploited for the development of plasmonic nanosensors. This is mainly due to their easy chemical degradation through oxidation, poor colloidal stability, and usually broad size distribution after synthesis, which leads to broad localized surface plasmon resonance bands. Coatings based on polymers such as poly(ethylene glycol) (PEG) or poly(vinylpyrrolidone) (PVP) and plant extracts have been used for the stabilization of AgNPs; however, these thick coatings are not suitable for sensing applications as they isolate the metallic core. The examples of stable AgNPs coated with a thin organic layer remain scarce in comparison to their gold counterparts. In this work, we present a convenient one-step synthesis strategy that allows to obtain unique gold, silver, and bimetallic NPs that combine all of the properties required for biosensing applications. The NPs are stabilized by a tunable calix[4]arene-based monolayer obtained through the reduction of calix[4]arene-tetradiazonium salts. These multidentate ligands are of particular interest as (i) they provide excellent colloidal and chemical stabilities to the particles thanks to their anchoring to the surface via multiple chemical bonds, (ii) they allow the subsequent (bio)conjugation of (bio)molecules under mild conditions, and (iii) they allow a control over the composition of mixed coating layers. Ag and Ag_Au nanoparticles of a high stability are obtained, opening perspectives for development of numerous biosensing applications.

Ultrastable PEGylated Calixarene-Coated Gold Nanoparticles with a Tunable Bioconjugation Density for Biosensing Applications

Many in vivo and in vitro applications using gold nanoparticles (AuNPs) require (i) their PEGylation, as it increases their stability and prevents nonspecific protein adsorption, and (ii) their conjugation to biomolecules, that provides them with specific recognition properties. Currently, the functionalization of AuNPs is based on thiol chemistry that suffers from two major drawbacks: (i) the Au-S bond is labile and confers limited chemical robustness to the organic layer, and (ii) control over the bioconjugation density is highly challenging. We report here a novel functionalization strategy based on calix[4]arene-tetradiazonium platforms for the coating of AuNPs with a robust PEG layer and their controlled bioconjugation. AuNPs were first modified with a functional calix[4]arene-diazonium salt bearing three PEG chains ended by a methoxy group and one by a carboxyl group. The resulting particles showed excellent chemical and colloidal stabilities, compared to similar systems obtained via a classical thiol chemistry, and could even be dispersed in human serum without degrading or aggregating. In addition to that, the carboxyl groups protruding from the PEG layer allowed their conjugation via amide bond formation with amine-containing biomolecules such as peptides. The control of the bioconjugation was obtained by grafting mixed layers of functional and nonfunctional PEGylated calix[4]arenes, that allowed varying the number of functional groups carried by the AuNPs and subsequently their bioconjugation capacity while preserving their dense protective PEG shell. Finally, we used these nanomaterials, modified with peptide aptamers, for the in vitro biosensing of a cancer biomarker, Mdm2.

2020

Adaptable DNA interactions regulate surface triggered self assembly

Exploiting DNA-mediated multivalent interactions, we demonstrate the selective triggering of colloidal self-assembly in the presence of a functional surface.

The role of microRNAs in ovarian function and the transition toward novel therapeutic strategies in fertility preservation: from bench to future clinical application

Abstract BACKGROUND New therapeutic approaches in oncology have converted cancer from a certain death sentence to a chronic disease. However, there are still challenges to be overcome regarding the off-target toxicity of many of these treatments. Oncological therapies can lead to future infertility in women. Given this negative impact on long-term quality of life, fertility preservation is highly recommended. While gamete and ovarian tissue cryopreservation are the usual methods offered, new pharmacological-based options aiming to reduce ovarian damage during oncological treatment are very attractive. In this vein, advances in the field of transcriptomics and epigenomics have brought small noncoding RNAs, called microRNAs (miRNAs), into the spotlight in oncology. MicroRNAs also play a key role in follicle development as regulators of follicular growth, atresia and steroidogenesis. They are also involved in DNA damage repair responses and they can themselves be modulated during chemotherapy. For these reasons, miRNAs may be an interesting target to develop new protective therapies during oncological treatment. This review summarizes the physiological role of miRNAs in reproduction. Considering recently developed strategies based on miRNA therapy in oncology, we highlight their potential interest as a target in fertility preservation and propose future strategies to make the transition from bench to clinic. OBJECTIVE AND RATIONALE How can miRNA therapeutic approaches be used to develop new adjuvant protective therapies to reduce the ovarian damage caused by cytotoxic oncological treatments? SEARCH METHODS A systematic search of English language literature using PubMed and Google Scholar databases was performed through to 2019 describing the role of miRNAs in the ovary and their use for diagnosis and targeted therapy in oncology. Personal data illustrate miRNA therapeutic strategies to target the gonads and reduce chemotherapy-induced follicular damage. OUTCOMES This review outlines the importance of miRNAs as gene regulators and emphasizes the fact that insights in oncology can inspire new adjuvant strategies in the field of onco-fertility. Recent improvements in nanotechnology offer the opportunity for drug development using next-generation miRNA-nanocarriers. WIDER IMPLICATIONS Although there are still some barriers regarding the immunogenicity and toxicity of these treatments and there is still room for improvement concerning the specific delivery of miRNAs into the ovaries, we believe that, in the future, miRNAs can be developed as powerful and non-invasive tools for fertility preservation.

2019

Limits of thiol chemistry revealed by quantitative analysis of mixed layers of thiolated-PEG ligands grafted onto gold nanoparticles

Kinetics of Nanoparticle-Membrane Adhesion Mediated by Multivalent Interactions

Multivalent adhesive interactions mediated by a large number of ligands and receptors underpin many biological processes, including cell adhesionand the uptake of particles, viruses, parasites, and nanomedical vectors. In materials science, multivalent interactions between colloidal particles have enabled unprecedented control over the phase behavior of self-assembled materials. Theoretical and experimental studies have pinpointed the relationship between equilibrium states and microscopic system parameters such as ligand-receptor binding strength and their density. In regimes of strong interactions, however, kinetic factors are expected to slow down equilibration, and lead to the emergence of long-lived out-of-equilibrium states that may significantly influence the outcome of self-assembly experiments and the adhesion of particles to biological membranes. Here we experimentally investigate the kinetics of adhesion of nanoparticles to biomimetic lipid membranes. Multivalent interactions are produced by strongly interacting DNA constructs, playing the role of both ligands and receptors. The rate of nanoparticle adhesion is investigated as a function of the surface density of membrane anchored receptors and of the bulk concentration of nanoparticles, and is observed to decrease substantially in regimes where the number of available receptors is limited compared to the overall number of ligands. We attribute such peculiar behaviour to the rapid sequestration of available receptors after initial nanoparticle adsorption. The experimental trends and the proposed interpretation are supported by numerical simulations.

2018

Submerging a Biomimetic Metallo-Receptor in Water for Molecular Recognition: Micellar Incorporation or Water Solubilization? A Case Study

Molecular recognition in water is an important topic, but a challenging task due to the very competitive nature of the medium. The focus of this study is the comparison of two different strategies for the water solubilization of a biomimetic metallo-receptor based on a poly(imidazole) resorcinarene core. The first relies on a new synthetic path for the introduction of hydrophilic substituents on the receptor, at a remote distance from the coordination site. The second involves the incorporation of the organosoluble metallo-receptor into dodecylphosphocholine (DPC) micelles, which mimic the proteic surrounding of the active site of metallo-enzymes. The resorcinarene ligand can be transferred into water through both strategies, in which it binds ZnII over a wide pH window. Quite surprisingly, very similar metal ion affinities, pH responses, and recognition properties were observed with both strategies. The systems behave as remarkable receptors for small organic anions in water at near-physiological pH. These results show that, provided the biomimetic site is well structured and presents a recognition pocket, the micellar environment has very little impact on either metal ion binding or guest hosting. Hence, micellar incorporation represents an easy alternative to difficult synthetic work, even for the binding of charged species (metal cations or anions), which opens new perspectives for molecular recognition in water, whether for sensing, transport, or catalysis.

The 3rd degree of biomimetism: associating the cavity effect, ZnII coordination and internal base assistance for guest binding and activation

The synthesis and characterization of a resorcinarene-based tetra(imidazole) ligand is reported. The properties of the corresponding ZnII complex are studied in depth, notably by NMR spectroscopy. In MeCN, acid-base titration reveals that one out of the four imidazole arms is hemi-labile and can be selectively protonated, thereby opening a coordination site in the exo position. Quite remarkably, the 4th imidazole arm promotes binding of an acidic molecule (a carboxylic acid, a β-diketone or acetamide), by acting as an internal base, which allows guest binding as an anion to the metal center in the endo position. Most importantly, the presence of this labile imidazole arm makes the ZnII complex active for the catalyzed hydration of acetonitrile. It is proposed that it acts as a general base for activating a water molecule in the vicinity of the metal center during its nucleophilic attack to the endo-bound MeCN substrate. This system presents a unique degree of biomimetism when considering zinc enzymes: a pocket for guest binding, a similar first coordination sphere, a coordination site available for water activation in the cis position relative to the substrate and finally an internal imidazole residue that plays the role of a general base.

2017

Temperature Dependence of Triplet-Triplet Annihilation Upconversion in Phospholipid Membranes

Understanding the temperature dependency of triplet-triplet annihilation upconversion (TTA-UC) is important for optimizing biological applications of upconversion. Here the temperature dependency of red-to-blue TTA-UC is reported in a variety of neutral PEGylated phospholipid liposomes. In these systems a delicate balance between lateral diffusion rate of the dyes, annihilator aggregation, and sensitizer self-quenching leads to a volcano plot, with the maximum upconversion intensity occurring near the main order-disorder transition temperature of the lipid membrane.

Versatile self-adapting boronic acids for H-bond recognition: from discrete to polymeric supramolecules

By taking advantage of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest has risen to use the aryl derivatives in materials science and supramolecular chemistry, nevertheless their ability to form H-bonded complexes has been only marginally touched. Herein we report the first solution and solid-state binding studies of first double H-bonded DD•AA-type complexes of a series of aromatic boronic acids that adopt a synsyn conformation with suitable complementary H-bonding acceptor partners. The first determination of the Ka in solution of ortho substituted boronic acids showed that 1:1 association is in the range between 300 and 6900 M-1. Crystallization of dimeric 1:1, trimeric 1:2 and 2:1 complexes enabled in depth examination of these complexes in the solid state, proving the selection of the -B(OH)2 syn-syn conformer through a pair of frontal H-bonds with the relevant AA partner. Non-ortho substituted boronic acids result in “flat” complexes. On the other hand, sterically demanding analogues bearing ortho-substituents strive to retain their recognition properties rotating the ArB(OH)2 moiety forming “T-shaped” complexes. Solid-state studies of a diboronic acid and a tetraazanaphthacene provided for the first time the formation of a supramolecular H-bonded polymeric ribbon. Given the conformational dynamicity of the -B(OH)2 functional group, it is expected that these findings will also open new possibilities in metal-free catalysis or organic crystal engineering, where double H-bonding donor boronic acids could act as suitable organocatalysts or templates developing functional materials with tailored organizational properties.

Controlled Functionalization of Gold Nanoparticles with Mixtures of Calix[4]arenes Revealed by Infrared Spectroscopy

Labile ligands such as thiols and carboxylates are commonly used to functionalize AuNPs, though little control over the composition is possible when mixtures of ligands are used. It was shown recently that robustly functionalized AuNPs can be obtained through the reductive grafting of calix[4]arenes bearing diazonium groups on the large rim. Here, we report a calix[4]arene-tetradiazonium decorated by four oligo(ethylene glycol) chains on the small rim, which upon grafting gave AuNPs with excellent stability thanks to the C-Au bonds. Mixtures of this calixarene and one with four carboxylate groups were grafted on AuNPs. The resulting particles were analyzed by infrared spectroscopy, which revealed that the composition of the ligand shell clearly reflected the ratio of calixarenes that was present in solution. This strategy opens the way to robustly protected AuNPs with well-defined numbers of functional or postfunctionalizable groups.

2016

Rapid and Selective Detection of Proteins by Dual Trapping Using Gold Nanoparticles Functionalized with Peptide Aptamers

A colorimetric platform for the fast, simple, and selective detection of proteins of medical interest is presented. Detection is based on the aggregation of two batches of peptide functionalized gold nanoparticles via the dual-trapping of the protein of interest. As proof of concept, we applied our platform to the detection of the oncoprotein Mdm2. The peptide aptamers used for the functionalization are based on the reported binding sequences of proteins p53 and p14 for the oncoprotein. Rapid aggregation, and a color change from red to purple, was observed upon addition of Mdm2 with concentrations as low as 20 nM. The selectivity of the system was demonstrated by the lack of response upon addition of bovine serum albumin (in large excess) or of a truncated version of Mdm2, which lacks one of the peptide binding sites. A linear response was observed between 30 and 50 nM of Mdm2. The platform reported here is flexible and can be adapted for the detection of other proteins when two binding peptide aptamers can be identified. Unlike current immunoassay methods, it is a one-step and rapid method with an easy readout signal and low production costs.

A comprehensive study to protein retention in hydrophobic interaction chromatography

The effect of different kosmotropic/chaotropic salt systems on retention characteristics of intact proteins has been examined in hydrophobic interaction chromatography (HIC). The performance was assessed using different column chemistries, i.e., polyalkylamide, alkylamine incorporating hydrophobic moieties, and a butyl chemistry. Selectivity in HIC is mainly governed by the salt concentration and by the molal surface tension increment of the salt. Typically, a linear relationship between the natural logarithm of the retention factor and the salt concentration is obtained. Using a 250 mm long column packed with 5 μm polyalkylamide functionalized silica particles and applying a 30 min linear salt gradient, a peak capacity of 78 was achieved, allowing the baseline separation of seven intact proteins. The hydrophobicity index appeared to be a good indicator to predict the elution order of intact proteins in HIC mode. Furthermore, the effect of adding additives in the mobile phase, such as calcium chloride (stabilizing the 3D conformation of α-lactalbumin) and isopropanol, on retention properties has been assessed. Results indicate that HIC retention is also governed by conformational in the proteins which affect the number of accessible hydrophobic moieties.

Extremely robust and post-functionalizable gold nanoparticles coated with calix[4]arenes via metal-carbon bonds

Gold nanoparticles stabilized with a thin layer of post-functionalizable calix[4]arenes were prepared through the reductive grafting of a calix[4]arene-tetra-diazonium salt. These particles show exceptional stability towards extreme pH, F-, NaCl, and upon drying. Post-functionalization of the calix-layer was demonstrated, opening the way to a wide range of applications.

Kinetic and Thermodynamic Stabilization of Metal Complexes by Introverted Coordination in a Calix[6]azacryptand

The Huisgen thermal reaction between an organic azide and an acetylene was employed for the selective monofunctionalization of a X6 -azacryptand ligand bearing a tren coordinating unit [X6 stands for calix[6]arene and tren for tris(2-aminoethyl)amine]. Supramolecular assistance, originating from the formation of a host-guest inclusion complex between the reactants, greatly accelerates the reaction while self-inhibition affords a remarkable selectivity. The new ligand possesses a single amino-leg appended at the large rim of the calixarene core and the corresponding Zn(2+) complex was characterized both in solution and in the solid state. The coordination of Zn(2+) not only involves the tren cap but also the introverted amino-leg, which locks the metal ion in the cavity. Compared with the parent ligand deprived of the amino-leg, the affinity of the new monofunctionalized X6 tren ligand 6 for Zn(2+) is found to have a 10-fold increase in DMSO, which is a very competitive solvent, and with an enhancement of at least three orders of magnitude in CDCl3 /CD3 OD (1:1, v/v). In strong contrast with the fast binding kinetics, decoordination of Zn(2+) as well as transmetallation appeared to be very slow processes. The monofunctionalized X6 tren ligand 6 fully protects the metal ion from the external medium thanks to the combination of a cavity and a closed coordination sphere, leading to greater thermodynamic and kinetic stabilities.

2015

Amino Acid Induced Fractal Aggregation of Gold Nanoparticles: Why and How

Gold colloids are the object of many studies as they are reported to have potential biological sensing, imaging and drug de-livery applications. In the presence of certain amino acids the aggregation of the gold nanoparticles into linear structures is observed, as highlighted by the appearance of a second plasmon band in the UV-Vis spectra of the colloid. The mechanism behind this phenomenon is still under debate. In order to help elucidate this issue, the interaction between gold colloids and different amino acids, modified amino acids and molecules mimicking their side-chain was monitored by UV-Vis absorp-tion, DLS and TEM. The results show that phenomenon can be rationalized in terms of the Diffusion Limited Colloid Ag-gregation (DLCA) model which gives rise to the fractal aggregation colloids. The global charge of the compound, which in-fluences the ionic strength of the solution, and the ease with which the compound can interact with the GNPs and affect their surface potential, are, the two parameters which control the DLCA regime. Calculations based on the Derjaguin, Lan-dau, Verwey and Overbeek (DLVO) theory confirm all the experimental observations.

Triplet-triplet annihilation upconversion followed by FRET for the red light activation of a photodissociative ruthenium complex in liposomes

Upconversion is a promising way to trigger high-energy photochemistry with low-energy photons. However, combining upconversion schemes with non-radiative energy transfer is challenging because bringing several photochemically active components in close proximity results in complex multi-component systems where quenching processes may deactivate the whole assembly. In this work, PEGylated liposomes were prepared that contained three photoactive components: a porphyrin dye absorbing red light, a perylene moiety emitting in the blue, and a light-activatable ruthenium prodrug sensitive to blue light. Time-dependent spectroscopic studies demonstrate that singlet perylene excited states are non-radiatively transferred to the nearby ruthenium complex by Förster resonance energy transfer (FRET). Under red-light irradiation of the three-component membranes, triplet-triplet annihilation upconversion (TTA-UC) occurs followed by FRET, which results in a more efficient activation of the ruthenium prodrug compared to a physical mixture of two-component upconverting liposomes and liposomes containing only the ruthenium complex. This work represents a rare example where TTA-UC and Förster resonance energy transfer are combined to achieve prodrug activation in the phototherapeutic window.

A biomimetic heteroditopic receptor for zwitterions in protic media

The efficient synthesis of calix[6]cryptothiourea 6 was achieved through a two-step sequence that involves a key [1+1] macrocyclization step. It was shown by NMR spectroscopy that this heteroditopic receptor can bind zwitterions in protic media with an outstanding selectivity for balanine betaine G5, which is likely due to a high complementarity between the two partners. This result constitutes a rare example of cavity complexation of a zwitterion by a calix[6]arene. In comparison with the parent urea-based receptors, 6 behaves as a much more efficient host for betaines. This strengthening of the binding properties is due to the better preorganization of the tripodal hydrogenbonding cap as well as to the higher acidity of the thiourea groups and their poor ability to self-associate. Remarkably, host 6 is able to perform solid-liquid as well as liquid-liquid extraction of G5. Finally, 6 provides an excellent structural model for the binding site of glycine betaine G4 encountered in natural systems.

2014

Thermodynamic study of the interaction between hen egg white lysozyme and Ce(IV)-Keggin polyoxotungstate as artificial protease

The molecular interactions of the Keggin polyoxometalate [Me2NH2]10[Ce(PW11O39)2] (1), which promotes selective hydrolysis of hen egg white lysozyme (HEWL) under physiological conditions, were investigated in detail by isothermal titration calorimetry (ITC), 31P NMR and circular dichroism (CD) spectroscopy. ITC experiments showed that mixing of 1 and HEWL at pH 7.4 and 25 or 37 °C resulted in complexes having 1 : 1 and 2 : 1 POM : HEWL stoichiometries, respectively, and thermodynamic profiles are in agreement with binding in the vicinity of the Trp28-Val29 and Asn44-Arg45 peptide bonds, which were previously shown to undergo selective hydrolysis by 1. Mixing of HEWL with (NH4)4Ce(SO4)4·4H2O salt indicated the absence of any binding accentuating the importance of the polyoxometalate scaffold for selective interaction with the HEWL surface. In contrast, the lacunary Na9[A-α-PW9O34] polyoxometalate showed an increased binding stoichiometry as compared to 1. Increasing the ionic strength resulted in thermodynamic signatures which indicate preservation of the interaction at the Trp28-Val29 site, while interaction at the Asn44-Arg45 appears disrupted due to competition with the salt ions. Decreasing the pH to 4.4 at 37 °C resulted in energetic contributions which suggest that binding at the Trp28-Val29 site is favored, while more pronounced binding at the Asn44-Arg45 site was anticipated when the pH was increased to 9.2. The absence of binding between 1 and α-lactalbumin (α-LA), a protein which is highly isostructural to HEWL but with an overall negative charge, was confirmed at pH 7.4 and 37 °C. The influence of the pH on the binding between 1 and α-LA was investigated, demonstrating that at lower pH values, where α-LA becomes more positively charged, a 1 : 1 interaction with 1 is observed. This journal is

Molecular origin of the hydrolytic activity and fixed regioselectivity of a Zr(IV)-substituted polyoxotungstate as artificial protease

A multitechnique approach has been applied in order to identify the thermodynamic and kinetic parameters related to the regioselective hydrolysis of human serum albumin (HSA) promoted by the Wells-Dawson polyoxometalate (POM), K15H[Zr(α2-P2W17O 61)2]. Isothermal titration calorimetry (ITC) studies indicate that up to four POM molecules interact with HSA. While the first interaction site is characterized by a 1:1 binding and an affinity constant of 2×108M-1, the three remaining sites are characterized by a lower global affinity constant of 7×10 5M-1. The higher affinity constant at the first site is in accordance with a high quenching constant of 2.2×108M -1 obtained for fluorescence quenching of the Trp214 residue located in the only positively charged cleft of HSA, in the presence of K 15H[Zr(α2-P2W17O 61)2]. In addition, EuIII luminescence experiments with an EuIII-substituted POM analogue have shown the replacement of water molecules in the first coordination sphere of Eu III due to binding of the metal ion to amino acid side chain residues of HSA. All three interaction studies are in accordance with a stronger POM dominated binding at the positive cleft on the one hand, and interaction mainly governed by metal anchoring at the three remaining positions, on the other hand. Hydrolysis experiments in the presence of K15H[Zr(α 2-P2W17O61)2] have demonstrated regioselective cleavage of HSA at the Arg114-Leu115, Ala257-Asp258, Lys313-Asp314 or Cys392-Glu393 peptide bonds. This is in agreement with the interaction studies as the Arg114-Leu115 peptide bond is located in the positive cleft of HSA and the three remaining peptide bonds are each located near an upstream acidic residue, which can be expected to coordinate to the metal ion. A detailed kinetic study has evidenced the formation of additional fragments upon prolonged reaction times. Edman degradation of the additional reaction products has shown that these fragments result from further hydrolysis at the initially observed cleavage positions, indicating a fixed selectivity for K 15H[Zr(α2-P2W17O 61)2]. Making the cut: A multitechnique approach has been applied to the identification of the thermodynamic and kinetic parameters related to the regioselective hydrolysis of human serum albumin promoted by the Wells-Dawson polyoxometalate (POM), K15H[Zr(α2- P2W17O61)2] (see figure). All interaction studies are in accordance with the observed hydrolysis fragments which are the result of combined POM-dominated binding and interactions governed by metal anchoring. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Binding of a ruthenium complex to a thioether ligand embedded in a negatively charged lipid bilayer: a two-step mechanism

The interaction between the ruthenium polypyridyl complex [Ru(terpy)(dcbpy)(H2O)]2+ (terpy=2,2;6,2"- terpyridine, dcbpy=6,6-dichloro-2,2-bipyridine) and phospholipid membranes containing either thioether ligands or cholesterol were investigated using UV-visible spectroscopy, Langmuir-Blodgett monolayer surface pressure measurements, and isothermal titration calorimety (ITC). When embedded in a membrane, the thioether ligand coordinated to the dicationic metal complex only when the phospholipids of the membrane were negatively charged, that is, in the presence of attractive electrostatic interaction. In such a case coordination is much faster than in homogeneous conditions. A two-step model for the coordination of the metal complex to the membrane-embedded sulfur ligand is proposed, in which adsorption of the complex to the negative surface of the monolayers or bilayers occurs within minutes, whereas formation of the coordination bond between the surface-bound metal complex and ligand takes hours. Finally, adsorption of the aqua complex to the membrane is driven by entropy. It does not involve insertion of the metal complex into the hydrophobic lipid layer, but rather simple electrostatic adsorption at the water-bilayer interface.© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fluorescent Chemosensors for Anions and Contact Ion Pairs with a Cavity-Based Selectivity

The association of a concave macrocyclic compound to one or multiple fluorophores is an appealing strategy for the design of chemosensors. Indeed, as with biological systems, a cavity-based selectivity can be expected with such fluorescent receptors. Examples of calix[6]arene-based systems using this strategy are rare in the literature, and to our knowledge, no examples of fluorescent receptors that can bind organic contact ion pairs have been reported. This report describes the straightforward synthesis of fluorescent calix[6]arene-based receptors 4a and 4b bearing three pyrenyl subunits and the study of their binding properties toward anions and ammonium salts using different spectroscopies. It was found that receptor 4a exhibits a remarkable selectivity for the sulfate anion in DMSO, enabling its selective sensing by fluorescence spectroscopy. In CDCl3, the receptor is able to bind ammonium ions efficiently only in association with the sulfate anion. Interestingly, this cooperative binding of ammonium sulfate salts was also evidenced in a protic environment. Finally, a cavity-based selectivity in terms of size and shape of the guest was observed with both receptors 4a and 4b, opening interesting perspectives on the elaboration of fluorescent cavity-based systems for the selective sensing of biologically relevant ammonium salts such as neurotransmitters.

2013

DNA-Promoted Auto-Assembly of Gold Nanoparticles: Effect of the DNA Sequence on the Stability of the Assemblies

The use of deoxyribonucleic acid (DNA) oligonucleotides has proven to be a powerful and versatile strategy to assemble nanomaterials into two (2D) and three-dimensional (3D) superlattices. With the aim of contributing to the elucidation of the factors that affect the stability of this type of superlattices, the assembly of gold nanoparticles grafted with different DNA oligonucleotides was characterized by UV-Vis absorption spectroscopy as a function of temperature. After establishing an appropriate methodology the effect of (i) the length of the grafted oligonucleotides; (ii) the length of their complementary parts and also of (iii) the simultaneous grafting of different oligonucleotides was investigated. Our results indicate that the electrostatic repulsion between the particles and the cooperativity of the assembly process play crucial roles in the stability of the assemblies while the grafting density of the oligonucleotide strands seems to have little influence.

UV-Vis and NMR study of the formation of gold nanoparticles by citrate reduction: Observation of gold-citrate aggregates

The citrate reduction of gold(III) in water is one of the most commonly used synthetic pathways for the preparation of gold colloids. In order to gain insight into the formation of gold nanoparticles (GNPs) using this method, the synthesis of GNPs was undertaken under different experimental conditions and monitored in operando by UV-Vis spectroscopy. These experiments highlight that citrate should be polydeprotonated and that Au(III) should not be polyhydroxylated in order to obtain GNPs with a narrow size distribution. Samples taken during the reaction were also characterized by Nuclear Magnetic Resonance Spectroscopy (NMR) to monitor the various reaction products as a function of time. Diffusion Ordered SpectroscopY (DOSY) experiments allowed us to identify slow diffusing citrate - Au(I) or Au(0) complexes which could play a role in the formation of GNPs. © 2013 Elsevier Inc.

2012

An integrated fragment based screening approach for the discovery of small molecule modulators of the VWF-GPIba interaction

An integrated approach comprising STD NMR screening, pharmacophore based analogue selection and a bioassay is presented for the discovery of a stabilizer of the clinically relevant VWF-GPIbα protein-protein interaction.

Monitoring Fluoride Binding in DMSO: Why is a Singular Binding Behavior Observed?

Fluoride recognition by molecular receptors is attracting much interest in the scientific community and a variety of approaches are used to achieve recognition with high affinity and selectivity. Most of the synthetic systems reported have been extensively studied in organic solvents by using tetrabutylammonium fluoride (TBAF) as the source of fluoride. In many cases, titration behaviors are observed that cannot be ascribed to a classical 1:1 binding isotherm. By using UV/Vis, 19F NMR, and 1H NMR spectroscopy we have been able to highlight that the equilibrium between the fluoride and the corresponding bihalide ion, HF2-, which is inevitably generated in situ due to trace amounts of water, can be at the origin of this singular behavior. Our results highlight that when undertaking titrations with fluoride in DMSO, the data can be affected by the fluoride-bihalide equilibrium and that the latter species can even be the dominating species at low TBAF concentrations.

2011

Prebiotic chemistry : A fuzzy field

If prebiotic chemistry is defined as the study of the chemical steps, which lead to the first organisms, a clear-cut definition of “living organism” is needed. Unfortunately, no unambiguous and universally accepted definition exists for the concept “living”. Under these conditions, fuzzy logic is probably the methodological tool that can best be used to handle questions pertaining to “the origin of life”. A conventional scale must, however, be defined which interestingly enough, depends necessarily on our present-day scientific knowledge.

2010

Is it useful to have a clear-cut definition of life? On the use of fuzzy logic in prebiotic chemistry.

Many scientists, including one of the authors of the present paper, have devoted time to try to find a definition for life (Bersini and Reisse 2007). It is clear that a consensus will never be reached but, more importantly, it seems that the issue itself could be without major interest. It is indeed impossible to define a "natural" frontier between non-living and living systems and therefore also impossible to define dichotomic criteria which could be used in order to classify systems in one of these two classes (living or non-living). Fuzzy logic provides a natural way to deal with problems where class membership lacks sharply defined criteria. It also offers the possibility to avoid losing time with unnecessary controversies such as deciding whether a virus is, or is not, a living system.

2009

Comparison of the thermodynamics and base-pair dynamics of a full LNA:DNA duplex and of the isosequential DNA:DNA duplex

Locked nucleic acids (LNA), conformationally restricted nucleotide analogues, are known to enhance pairing stability and selectivity toward complementary strands. With the aim to contribute to a better understanding of the origin of these effects, the structure, thermal stability, hybridization thermodynamics, and base-pair dynamics of a full-LNA:DNA heteroduplex and of its isosequential DNA:DNA homoduplex were monitored and compared. CD measurements highlight differences in the duplex structures: the homoduplex and heteroduplex present B-type and A-type helical conformations, respectively. The pairing of the hybrid duplex is characterized, at all temperatures monitored (between 15 and 37 degrees C), by a larger stability constant but a less favorable enthalpic term. A major contribution to this thermodynamic profile emanates from the presence of a hairpin structure in the LNA single strand which contributes favorably to the entropy of interaction but leads to an enthalpy penalty upon duplex formation. The base-pair opening dynamics of both systems was monitored by NMR spectroscopy via imino protons exchange measurements. The measurements highlight that hybrid G-C base-pairs present a longer base-pair lifetime and higher stability than natural G-C base-pairs, but that an LNA substitution in an A-T base-pair does not have a favorable effect on the stability. The thermodynamic and dynamic data confirm a more favorable stacking of the bases in the hybrid duplex. This study emphasizes the complementarities between dynamic and thermodynamical studies for the elucidation of the relevant factors in binding events.

15N NMR relaxation data reveal significant chemical exchange broadening in the alpha-domain of human α-lactalbumin

Human alpha-lactalbumin (alpha-LA), a 123-residue calcium-binding protein, has been studied using (15)N NMR relaxation methods in order to characterize backbone dynamics of the native state at the level of individual residues. Relaxation data were collected at three magnetic field strengths and analyzed using the model-free formalism of Lipari and Szabo. The order parameters derived from this analysis are generally high, indicating a rigid backbone. A total of 46 residues required an exchange contribution to T(2); 43 of these residues are located in the alpha-domain of the protein. The largest exchange contributions are observed in the A-, B-, D-, and C-terminal 3(10)-helices of the alpha-domain; these residues have been shown previously to form a highly stable core in the alpha-LA molten globule. The observed exchange broadening, along with previous hydrogen/deuterium amide exchange data, suggests that this part of the alpha-domain may undergo a local structural transition between the well-ordered native structure and a less-ordered molten-globule-like structure.

2008

Catalysis by mettalloenzymes and the origin of life

2007

Protonation linked equilibria and apparent affinity constants: the thermodynamic profile of the alpha-chymotrypsin-proflavin interaction

Protonation/deprotonation equilibria are frequently linked to binding processes involving proteins. The presence of these thermodynamically linked equilibria affects the observable thermodynamic parameters of the interaction (K(obs), DeltaH(obs)(0) ). In order to try and elucidate the energetic factors that govern these binding processes, a complete thermodynamic characterisation of each intrinsic equilibrium linked to the complexation event is needed and should furthermore be correlated to structural information. We present here a detailed study, using NMR and ITC, of the interaction between alpha-chymotrypsin and one of its competitive inhibitors, proflavin. By performing proflavin titrations of the enzyme, at different pH values, we were able to highlight by NMR the effect of the complexation of the inhibitor on the ionisable residues of the catalytic triad of the enzyme. Using ITC we determined the intrinsic thermodynamic parameters of the different equilibria linked to the binding process. The possible driving forces of the interaction between alpha-chymotrypsin and proflavin are discussed in the light of the experimental data and on the basis of a model of the complex. This study emphasises the complementarities between ITC and NMR for the study of binding processes involving protonation/deprotonation equilibria.

Developments in the characterisation of the catalytic triad of alpha-chymotrypsin: Effect of the protonation state of Asp102 on the 1H NMR signals of His57

Protonated or not? 1H NMR spectra of α-chymotrypsin were recorded as a function of pH (from top to bottom pH 8.6, 5.1 and 4.2). Slow exchange is observed for the NH protons of His57 between two environments due to different protonation states of Asp102. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

2005

Differential scanning calorimetry in life science: thermodynamics, stability, molecular recognition and application in drug design

All biological phenomena depend on molecular recognition, which is either intermolecular like in ligand binding to a macromolecule or intramolecular like in protein folding. As a result, understanding the relationship between the structure of proteins and the energetics of their stability and binding with others (bio)molecules is a very interesting point in biochemistry and biotechnology. It is essential to the engineering of stable proteins and to the structure-based design of pharmaceutical ligands. The parameter generally used to characterize the stability of a system (the folded and unfolded state of the protein for example) is the equilibrium constant (K) or the free energy (deltaG(o)), which is the sum of enthalpic (deltaH(o)) and entropic (deltaS(o)) terms. These parameters are temperature dependent through the heat capacity change (deltaCp). The thermodynamic parameters deltaH(o) and deltaCp can be derived from spectroscopic experiments, using the van't Hoff method, or measured directly using calorimetry. Along with isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC) is a powerful method, less described than ITC, for measuring directly the thermodynamic parameters which characterize biomolecules. In this article, we summarize the principal thermodynamics parameters, describe the DSC approach and review some systems to which it has been applied. DSC is much used for the study of the stability and the folding of biomolecules, but it can also be applied in order to understand biomolecular interactions and can thus be an interesting technique in the process of drug design.

Utilisation de la Calorimétrie à Titrage Isotherme pour l'étude des interactions entre (bio)molécules

2003

Comparison of the NMR enantiodifferentiation of a chiral ruthenium(II) complex of C2 symmetry using the TRISPHAT anion and a lanthanide shift reagent

The tris[tetrachlorobenzenediolato]phosphate(v) anion (TRISPHAT) is known to be an efficient NMR chiral shift agent for various chiral cationic species. Here we compare the efficiency of TRISPHAT and of a chiral lanthanide shift reagent for the determination of the enantiomeric purity of the chiral building block [Ru(phen)[2]PY[2]][2][+] which possesses C[2] symmetry. We also discuss our results in terms of the geometry of interaction between the Ru(II) complex and the TRISPHAT anion.

Articles dans des revues avec comité de lecture

2023

The Lucigenin Assay: Measuring Anion Transport in Lipid Vesicles

Synthetic anion transporters are developed to transport anions across lipid membranes with the long-term perspective of biological applications. The lucigenin assay is a popular tool to study their transport of chloride and other anions in liposomes. It relies on the quenching of the fluorescence of encapsulated lucigenin by anions, which can be monitored by fluorescence spectroscopy. This article provides a tutorial introduction to the practical use and understanding of the lucigenin assay. It describes in detail how to use this assay to monitor chloride/nitrate antiport in liposomes, process and interpret the data, and solve common issues. Variations of the assay enabling the investigation of the transport of other anions and transport mechanisms are discussed. Furthermore, a zwitterionic analogue of lucigenin is demonstrated to have advantages for use in experiments over longer time scales, as it does not leak out of the liposomes, or when studying chloride uniport, as it avoids interference from antiport with nitrate that is present in commercial lucigenin.

Transmembrane Transport of Inorganic Phosphate by a Strapped Calix[4]pyrrole

Synthetic anion receptors are increasingly being explored for the transport of anions across lipid membranes because of their potential therapeutic applications. A considerable amount of research focuses on the transport of chloride, whereas the transmembrane transport of inorganic phosphate has not been reported to date, despite the biological relevance of this anion. Here we present a calix[4]pyrrole with a bisurea strap that functions as a receptor and transporter for H2PO4-, relying on the formation of eight hydrogen bonds and efficient encapsulation of the anion. Using a phosphate-sensitive lanthanide probe and 31P NMR spectroscopy, we demonstrate that this receptor can transport phosphate into vesicles by H2PO4-/Cl- antiport, H2PO4- uniport, and Cs+/H2PO4- symport mechanisms. This first example of inorganic phosphate transport by a neutral receptor opens perspectives for the future development of transporters for various biological phosphates.

Transmembrane transport of fluoride studied by time-resolved emission spectroscopy

Here we present a new method to monitor fluoride transmembrane transport into liposomes using a europium(III) complex. We take advantage of the long emission lifetime of this probe to measure the transport activity of a fluorescent transporter. The high sensitivity, selectivity, and versatility of the assay allowed us to study different types of fluoride transporters and unravel their mechanisms of action.

2022

Transmembrane Transport of Bicarbonate by Anion Receptors

The development of synthetic anion transporters is motivated by their potential application as treatment for diseases that originate from deficient anion transport by natural proteins. Transport of bicarbonate is important for crucial biological functions such as respiration and digestion. Despite this biological relevance, bicarbonate transport has not been as widely studied as chloride transport. Herein we present an overview of the synthetic receptors that have been studied as bicarbonate transporters, together with the different assays used to perform transport studies in large unilamellar vesicles. We highlight the most active transporters and comment on the nature of the functional groups present in active and inactive compounds. We also address recent mechanistic studies that have revealed different processes that can lead to net transport of bicarbonate, as well as studies reported in cells and tissues, and comment on the key challenges for the further development of bicarbonate transporters.

Dynamic covalent chemistry with azines

Azines provide an attractive balance in dynamic covalent chemistry, with rapid exchange in acidic conditions and good stability in the presence of water.

Dissecting transmembrane bicarbonate transport by 1,8-di(thio)amidocarbazoles

Synthetic ionophores able to transport bicarbonate and chloride anions across lipid bilayers are appealing for their wide range of potential biological applications. We have studied the bicarbonate and chloride transport by carbazoles with two amido/thioamido groups using a bicarbonate-sensitive europium(iii) probe in liposomes and found a highly remarkable transporter concentration dependence. This can be explained by a combination of two distinct transport mechanisms: HCO3−/Cl− exchange and a combination of unassisted CO2 diffusion and HCl transport, of which the respective contributions were quantified. The compounds studied were found to be highly potent HCl transporters. Based on the mechanistic insights on anion transport, we have tested the antimicrobial activity of these compounds and found a good correlation with their ion transport properties and a high activity against Gram-positive bacteria.

Monofunctionalized Fluorinated Bambusurils and Their Conjugates for Anion Transport and Extraction

Bambusurils are macrocyclic molecules that are known for their high binding affinity and selectivity toward anions. Here, we present the preparation of two bambusurils bearing fluorinated substituents and one carboxylic function. These monofunctionalized bambusurils were conjugated with crown ether and cholesterol units. The resulting conjugates were successfully tested in liquid-liquid extraction of inorganic salts and chloride/bicarbonate transport across lipid bilayers.

Calix[6]arenes with halogen bond donor groups as selective and efficient anion transporters

2021

Hydrazones in anion transporters: the detrimental effect of a second binding site

Synthetic anion transporters can be developed using anion receptors that are able to bind the anion and stabilize it in the lipophilic interior of a bilayer membrane, and they usually contain functional groups with acidic NHs, such as ureas, thioureas and squaramides. To assess the suitability of acylhydrazones as a new functional group for the preparation of anion transporters, we have studied a family of thioureas functionalized with these and related functional groups. 1H NMR titrations and DFT calculations indicate that the thioureas bearing acylhydrazone groups behave as chloride receptors with two separate binding sites, of which the acylhydrazone binds weaker than the thiourea. Chloride transport studies show that the additional binding site has a detrimental effect on thiourea-based transporters, and this phenomenon is also observed for bis(thio)ureas with two separate binding sites. We propose that the presence of a second anion binding unit hinders the transport activity of the thiourea due to additional interactions with the phospholipids of the membrane. In agreement with this hypothesis, extensive molecular dynamics simulations suggest that the molecules will tend to be positioned in the water/lipid interface, driven by the interaction of the NHs of the thiourea and of the acylhydrazone groups with the POPC polar head groups and water molecules. Moreover, the interaction energies show that the poorest transporters have indeed the strongest interactions with the membrane phospholipids, inhibiting chloride transport. This detrimental effect of additional functional groups on transport activity should be considered when designing new ion transporters, unless these groups cooperatively promote anion recognition and transmembrane transport.

Transmembrane Transport of Bicarbonate Unravelled

Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of particular interest, as defects in transmembrane transport of bicarbonate are associated with various diseases. However, no convenient method exists to directly observe bicarbonate transport and study the mechanisms involved. Here, an assay is presented that allows the kinetics of bicarbonate transport into liposomes to be monitored directly and with great sensitivity. The assay utilises an encapsulated europium(III) complex, which exhibits a large increase in emission intensity upon binding bicarbonate. Mechanisms involving CO2 diffusion and the dissipation of a pH gradient are shown to be able to lead to an increase in bicarbonate concentration within liposomes, without transport of the anion occurring at all. By distinguishing these alternative mechanisms from actual bicarbonate transport, this assay will inform the future development of bicarbonate transporters.

Hexagonal Microparticles from Hierarchical Self-Organization of Chiral Trigonal Pd3L6 Macrotetracycles

Construction of structurally complex architectures using inherently chiral, asymmetric, or multi-heterotopic ligands is a major challenge in metallosupramolecular chemistry. Moreover, the hierarchical self-organization of such complexes is unique. Here, we introduce a water-soluble, facially amphiphilic, amphoteric, chiral, asymmetric, and hetero-tritopic ligand derived from natural bile acid, ursodeoxycholic acid. We show that via the supramolecular transmetalation reaction, using nitrates of Cu(II) or Fe(III), and subsequently Pd(II), a superchiral Pd3L6 complex can be obtained. Even though several possible constitutional isomers of Pd3L6 could be formed, because of the ligand asymmetry and relative flexibility of carbamate-pyridyl moieties attached to the steroid scaffold, only a single product with C3 rotational symmetry was obtained. Finally, we demonstrate that these amphiphilic complexes can self-organize into hexagonal microparticles in aqueous media. This finding may lead to the development of novel self-assembled metal-organic functional materials made of natural, abundant, and relatively inexpensive steroidal compounds.

2020

Transmembrane transport of copper(I) by imidazole-functionalised calix[4]arenes

Here we present the first synthetic transmembrane transporters for Cu+. Calix[4]arenes with two imidazole groups have a linear coordination motif, which allows selective extraction of Cu+ into chloroform. Transmembrane transport of Cu+ into liposomes was investigated with a newly developed assay and the results open the way to biomedical applications of these Cu+ ionophores.

Synthesis and Binding Properties of a Tren-Capped Hexahomotrioxacalix[3]arene

The straightforward synthesis of a new hexahomotrioxacalix[3]arene‐based ligand capped by a tren subunit was developed and the binding properties of the corresponding zinc complex were explored by NMR spectroscopy. Similarly to the closely related calix[6]tren‐based systems, the homooxacalixarene core ensures the mononuclearity of the zinc complex and the metal center displays a labile coordination site for exogenous guests. However, very different host-guest properties were observed: i) in CDCl3, the zinc complex strongly binds a water molecule and is reluctant to recognize other neutral guests, ii) in CD3CN, the exo‐coordination of anions prevails. Thus, in strong contrast to the calix[6]tren‐based systems, the coordination of neutral guests that thread through the small rim and fill the polyaromatic cavity was not observed. This unique behaviour is likely due to the fact that the 18‐membered ethereal macrocycle is too small to let a molecule threading through it. This work illustrates the key role played by the second coordination sphere in the binding properties of metal complexes.

2019

Anion carriers as potential treatments for cystic fibrosis: transport in cystic fibrosis cells, and additivity to channel-targeting drugs

Synthetic anion transporters are active in cystic fibrosis cells, and are additive to clinically-approved drugs, suggesting new combination therapies for this lethal genetic condition.

Limits of thiol chemistry revealed by quantitative analysis of mixed layers of thiolated-PEG ligands grafted onto gold nanoparticles

Repositioning Chloride Transmembrane Transporters: Transport of Organic Ion Pairs

Given the biological importance of organic cations, the facilitated transport of organic ion pairs could find many applications. Calix[6]arene tris(thio)ureas, which possess a cavity that can accommodate primary ammonium ions, can not only act as carriers for Cl − /NO 3 − antiport but can also perform the cotransport of PrNH 3 Cl. Transport was monitored by fluorescence spectroscopy and the presence of the different species inside the vesicles was characterized by 1 H and 35 Cl NMR experiments involving shift reagents. The cotransport of PrNH 3 Cl was also observed by receptors deprived of a cavity, but the presence of the cavity conveys an advantage, as the cotransport by calix[6]arenes was observed to be more efficient than the Cl − /NO 3 − antiport, which is not the case with receptors without a cavity. The role played by the cavity was further highlighted by the disappearance of this advantage when using a bulky ammonium ion, which cannot be complexed within the cavity.

Fluorinated Bambusurils as Highly Effective and Selective Transmembrane Cl-/HCO3- Antiporters

The exchange of chloride and bicarbonate across lipid bilayers is an important biological process. Synthetic molecules can act as mobile carriers for these anions, although most show little selectivity. Here we report on three bambus[6]uril macrocycles functionalized with fluorinated benzyl groups, which are able to exchange Cl− and HCO3− efficiently. Remarkably, rates for Cl−/NO3− exchange are two orders of magnitude lower. The higher rates of Cl−/HCO3− transport can be explained by the ability of the bambusurils to complex Cl− and HCO3− simultaneously, facilitating their exchange at the bilayer interface. Furthermore, the exceptionally high affinity and selectivity of these systems for NO3− appear to contribute to the poor Cl−/NO3− exchange. This work not only demonstrates the importance of anion binding characteristics on anion transport but also the potential relevance of bambusurils for anion transport applications considering the high rate observed for Cl−/HCO3− exchange.

2018

Anion Recognition by a Bioactive Diureidodecalin Anionophore: Solid-State, Solution, and Computational Studies

Recent work has identified a bis-(p-nitrophenyl)ureidodecalin anion carrier as a promising candidate for biomedical applications, showing good activity for chloride transport in cells yet almost no cytotoxicity. To underpin further development of this and related compounds, a detailed structural and binding investigation is reported. Crystal structures of the transporter as five solvates confirm the diaxial positioning of urea groups while revealing a degree of conformational flexibility. Structures of complexes with Cl−, Br−, NO3 −, SO4 2− and AcO−, supported by computational studies, show how the binding site can adapt to accommodate these anions. 1H NMR binding studies revealed exceptionally high affinities for anions in DMSO, decreasing in the order SO4 2−>H2PO4 −≈HCO3 −≈AcO−≫HSO4 −>Cl−>Br−>NO3 −>I−. Analysis of the binding results suggests that selectivity is determined mainly by the H-bond acceptor strength of different anions, but is also modulated by receptor geometry.

Grafting of Oligo(ethylene glycol)-Functionalized Calix[4]arene-Tetradiazonium Salts for Antifouling Germanium and Gold Surfaces

Biosensors that can determine protein concentration and structure are highly desired for biomedical applications. For the development of such biosensors, the use of Fourier transform infrared (FTIR) spectroscopy with the attenuated internal total reflection (ATR) configuration is particularly attractive, but it requires appropriate surface functionalization of the ATR optical element. Indeed, the surface has to specifically interact with a target protein in close contact with the optical element and must display antifouling properties to prevent nonspecific adsorption of other proteins. Here, we report robust monolayers of calix[4]arenes bearing oligo(ethylene glycol) (oEG) chains, which were grafted on germanium and gold surfaces via their tetradiazonium salts. The formation of monolayers of oEGylated calix[4]arenes was confirmed by AFM, IR, and contact angle measurements. The antifouling properties of these modified surfaces were studied by ATR-FTIR spectroscopy and fluorescence microscopy, and the nonspecific absorption of bovine serum albumin was found to be reduced by 85% compared to that of unmodified germanium. In other words, the organic coating by oEGylated calix[4]arenes provides remarkable antifouling properties, opening the way for the design of germanium- or gold-based biosensors.

Anthracene Bisureas as Powerful and Accessible Anion Carriers

Anion transport by ortho-phenylene bis-ureas across cell and vesicle membranes

Ortho-Phenylene bis-ureas serve as anionophores in cells expressing halide-sensitive yellow fluorescent protein, as well as in synthetic vesicles. Activities can reach high levels, and are strongly dependent on the deliverability of the transporters.

2017

A folding decalin tetra-urea for transmembrane anion transport

A new approach to anionophore design combines a trans-decalin scaffold with flexible o-phenylene bis-urea side-arms. The scaffold preorganises the side-arms, which can fold to encompass anionic substrates. A prototype has been synthesised by coupling an o-aminophenyl urea to a decalin bis-isocyanate, and has been shown to be active for anion binding and transport. Modelling and NMR studies show that the unbound receptor is capable of intramolecular hydrogen bonding but can reorganise to form up to eight hydrogen bonds to a chloride anion.

Controlled Functionalization of Gold Nanoparticles with Mixtures of Calix[4]arenes Revealed by Infrared Spectroscopy

Labile ligands such as thiols and carboxylates are commonly used to functionalize AuNPs, though little control over the composition is possible when mixtures of ligands are used. It was shown recently that robustly functionalized AuNPs can be obtained through the reductive grafting of calix[4]arenes bearing diazonium groups on the large rim. Here, we report a calix[4]arene-tetradiazonium decorated by four oligo(ethylene glycol) chains on the small rim, which upon grafting gave AuNPs with excellent stability thanks to the C-Au bonds. Mixtures of this calixarene and one with four carboxylate groups were grafted on AuNPs. The resulting particles were analyzed by infrared spectroscopy, which revealed that the composition of the ligand shell clearly reflected the ratio of calixarenes that was present in solution. This strategy opens the way to robustly protected AuNPs with well-defined numbers of functional or postfunctionalizable groups.

2016

Nonprotonophoric Electrogenic Cl- Transport Mediated by Valinomycin-like Carriers

Synthetic transmembrane anion transporters (anionophores) have potential as tools for biomedical research and as therapeutic agents for diseases associated with anion-channel dysfunction. However, the possibility of H+ or OH- transport by anionophores has received little attention, and an anionophore selective for Cl- over H+/OH- is currently unavailable. Here, we show that depending on anionophore acidity, many anionophores facilitate electrogenic H+ or OH- transport, potentially leading to toxicity. Nevertheless, using several liposome-membrane-based assays, we identified two newly developed small molecules that promote electrogenic Cl- transport without effectively dissipating the transmembrane pH gradient, essentially mimicking the electrogenic cationophore valinomycin. The Cl- > H+/OH- selectivity of anionophores showed a consistent positive correlation with the degree of Cl- encapsulation and a negative correlation with the acidity of hydrogen-bond donors. Our study demonstrates that a valinomycin equivalent for Cl--selective transport is achievable.

Rapid and Selective Detection of Proteins by Dual Trapping Using Gold Nanoparticles Functionalized with Peptide Aptamers

A colorimetric platform for the fast, simple, and selective detection of proteins of medical interest is presented. Detection is based on the aggregation of two batches of peptide functionalized gold nanoparticles via the dual-trapping of the protein of interest. As proof of concept, we applied our platform to the detection of the oncoprotein Mdm2. The peptide aptamers used for the functionalization are based on the reported binding sequences of proteins p53 and p14 for the oncoprotein. Rapid aggregation, and a color change from red to purple, was observed upon addition of Mdm2 with concentrations as low as 20 nM. The selectivity of the system was demonstrated by the lack of response upon addition of bovine serum albumin (in large excess) or of a truncated version of Mdm2, which lacks one of the peptide binding sites. A linear response was observed between 30 and 50 nM of Mdm2. The platform reported here is flexible and can be adapted for the detection of other proteins when two binding peptide aptamers can be identified. Unlike current immunoassay methods, it is a one-step and rapid method with an easy readout signal and low production costs.

Tilting and Tumbling in Transmembrane Anion Carriers: Activity Tuning through n-Alkyl Substitution.

Anion transport by synthetic carriers (anionophores) holds promise for medical applications, especially the treatment of cystic fibrosis. Among the factors which determine carrier activity, the size and disposition of alkyl groups is proving remarkably important. Herein we describe a series of dithioureidodecalin anionophores, in which alkyl substituents on one face are varied from C0 to C10 in two-carbon steps. Activities increase then decrease as the chain length grows, peaking quite sharply at C6 . Molecular dynamics simulations showed the transporter chloride complexes releasing chloride as they approach the membrane-aqueous interface. The free transporter then stays at the interface, adopting an orientation that depends on the alkyl substituent. If chloride release is prevented, the complex is positioned similarly. Longer chains tilt the binding site away from the interface, potentially freeing the transporter or complex to move through the membrane. However, chains which are too long can also slow transport by inhibiting movement, and especially reorientation, within the phospholipid bilayer.

Targeted anion transporter delivery by coiled-coil driven membrane fusion

Synthetic anion transporters (anionophores) have potential as biomedical research tools and therapeutics. However, the efficient and specific delivery of these highly lipophilic molecules to a target cell membrane is non-trivial. Here, we investigate the delivery of a powerful anionophore to artificial and cell membranes using a coiled-coil-based delivery system inspired by SNARE membrane fusion proteins. Incorporation of complementary lipopeptides into the lipid membranes of liposomes and cell-sized giant unilamellar vesicles (GUVs) facilitated the delivery of a powerful anionophore into GUVs, where its anion transport activity was monitored in real time by fluorescence microscopy. Similar results were achieved using live cells engineered to express a halide-sensitive fluorophore. We conclude that coiled-coil driven membrane fusion is a highly efficient system to deliver anionophores to target cell membranes.

Extremely robust and post-functionalizable gold nanoparticles coated with calix[4]arenes via metal-carbon bonds

Gold nanoparticles stabilized with a thin layer of post-functionalizable calix[4]arenes were prepared through the reductive grafting of a calix[4]arene-tetra-diazonium salt. These particles show exceptional stability towards extreme pH, F-, NaCl, and upon drying. Post-functionalization of the calix-layer was demonstrated, opening the way to a wide range of applications.

Efficient, non-toxic anion transport by synthetic carriers in cells and epithelia.

Transmembrane anion transporters (anionophores) have potential for new modes of biological activity, including therapeutic applications. In particular they might replace the activity of defective anion channels in conditions such as cystic fibrosis. However, data on the biological effects of anionophores are scarce, and it remains uncertain whether such molecules are fundamentally toxic. Here, we report a biological study of an extensive series of powerful anion carriers. Fifteen anionophores were assayed in single cells by monitoring anion transport in real time through fluorescence emission from halide-sensitive yellow fluorescent protein. A bis-(p-nitrophenyl)ureidodecalin shows especially promising activity, including deliverability, potency and persistence. Electrophysiological tests show strong effects in epithelia, close to those of natural anion channels. Toxicity assays yield negative results in three cell lines, suggesting that promotion of anion transport may not be deleterious to cells. We therefore conclude that synthetic anion carriers are realistic candidates for further investigation as treatments for cystic fibrosis.

2015

Sterically geared tris-thioureas; transmembrane chloride transporters with unusual activity and accessibility.

Tris-N-arylthioureas derived in one step from 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene are remarkably effective anion carriers. With optimised aryl substituents their activities come close to the best currently known, suggesting that they might find use as readily available standards in anion transport research.

Electric-Field Control of Interfering Transport Pathways in a Single-Molecule Anthraquinone Transistor.

It is understood that molecular conjugation plays an important role in charge transport through single-molecule junctions. Here, we investigate electron transport through an anthraquinone based single-molecule three-terminal device. With the use of an electric-field induced by a gate electrode, the molecule is reduced resulting into a 10-fold increase in the off-resonant differential conductance. Theoretical calculations link the change in differential conductance to a reduction-induced change in conjugation, thereby lifting destructive interference of transport pathways.

Biotin[6]uril Esters: Chloride-Selective Transmembrane Anion Carriers Employing C-H···Anion Interactions.

Biotin[6]uril hexaesters represent a new class of anionophores which operate solely through C-H···anion interactions. The use of soft H-bond donors favors the transport of less hydrophilic anions (e.g., Cl(-), NO3(-)) over hard, stongly hydrated anions (e.g., HCO3(-) and SO4(2-)). Especially relevant is the selectivity between chloride and bicarbonate, the major inorganic anions in biological systems.

High-affinity anion binding by steroidal squaramide receptors.

Exceptionally powerful anion receptors have been constructed by placing squaramide groups in axial positions on a steroidal framework. The steroid preorganizes the squaramide NH groups such that they can act cooperatively on a bound anion, while maintaining solubility in nonpolar media. The acidic NH groups confer higher affinities than previously-used ureas or thioureas. Binding constants exceeding 10(14)  M(-1) have been measured for tetraethylammonium salts in chloroform by employing a variation of Cram's extraction procedure. The receptors have also been studied as transmembrane anion carriers in unilamellar vesicles. Unusually their activities do not correlate with anion affinities, thus suggesting an upper limit for binding strength in the design of anion carriers.

Visualization and quantification of transmembrane ion transport into giant unilamellar vesicles.

Transmembrane ion transporters (ionophores) are widely investigated as supramolecular agents with potential for biological activity. Tests are usually performed in synthetic membranes that are assembled into large unilamellar vesicles (LUVs). However transport must be followed through bulk properties of the vesicle suspension, because LUVs are too small for individual study. An alternative approach is described whereby ion transport can be revealed and quantified through direct observation. The method employs giant unilamellar vesicles (GUVs), which are 20-60 μm in diameter and readily imaged by light microscopy. This allows characterization of individual GUVs containing transporter molecules, followed by studies of transport through fluorescence emission from encapsulated indicators. The method provides new levels of certainty and relevance, given that the GUVs are similar in size to living cells. It has been demonstrated using a highly active anion carrier, and should aid the development of compounds for treating channelopathies such as cystic fibrosis.

2014

Large negative differential conductance in single-molecule break junctions.

Molecular electronics aims at exploiting the internal structure and electronic orbitals of molecules to construct functional building blocks. To date, however, the overwhelming majority of experimentally realized single-molecule junctions can be described as single quantum dots, where transport is mainly determined by the alignment of the molecular orbital levels with respect to the Fermi energies of the electrodes and the electronic coupling with those electrodes. Particularly appealing exceptions include molecules in which two moieties are twisted with respect to each other and molecules in which quantum interference effects are possible. Here, we report the experimental observation of pronounced negative differential conductance in the current-voltage characteristics of a single molecule in break junctions. The molecule of interest consists of two conjugated arms, connected by a non-conjugated segment, resulting in two coupled sites. A voltage applied across the molecule pulls the energy of the sites apart, suppressing resonant transport through the molecule and causing the current to decrease. A generic theoretical model based on a two-site molecular orbital structure captures the experimental findings well, as confirmed by density functional theory with non-equilibrium Green's functions calculations that include the effect of the bias. Our results point towards a conductance mechanism mediated by the intrinsic molecular orbitals alignment of the molecule.

Preorganized bis-thioureas as powerful anion carriers: chloride transport by single molecules in large unilamellar vesicles.

Transmembrane anion carriers (anionophores) have potential in biological research and medicine, provided high activities can be obtained. There is particular interest in treating cystic fibrosis (CF), a genetic illness caused by deficient anion transport. Previous work has found that anionophore designs featuring axial ureas on steroid and trans-decalin scaffolds can be especially effective. Here we show that replacement of ureas by thioureas yields substantial further enhancements. Six new bis-thioureas have been prepared and tested for Cl(-)/NO3(-) exchange in 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol large unilamellar vesicles (LUVs). The bis-thioureas are typically >10 times more effective than the corresponding ureas and are sufficiently active that transport by molecules acting singly in LUVs is readily detected. The highest activity is shown by decalin 9, which features N-(3,5-bis(trifluoromethyl)phenyl)thioureido and octyl ester substituents. A single molecule of transporter 9 in a 200 nm vesicle promotes Cl(-)/NO3(-) exchange with a half-life of 45 s and an absolute rate of 850 chloride anions per second. Weight-for-weight, this carrier is only slightly less effective than CFTR, the natural anion channel associated with CF.

Cross-conjugation and quantum interference: a general correlation?

We discuss the relationship between the π-conjugation pattern, molecular length, and charge transport properties of molecular wires, both from an experimental and a theoretical viewpoint. Specifically, we focus on the role of quantum interference in the conductance properties of cross-conjugated molecules. For this, we compare experiments on two series of dithiolated wires. The first set we synthesized consists of three dithiolated oligo(phenylene ethynylene) (OPE) benchmark compounds with increasing length. The second series synthesized comprises three molecules with different π-conjugation patterns, but identical lengths, i.e. an anthracene (linear conjugation), an anthraquinone (cross-conjugation), and a dihydroanthracene (broken conjugation) derivative. To benchmark reliable trends, conductance experiments on these series have been performed by various techniques. Here, we compare data obtained by conductive-probe atomic force microscopy (CP-AFM) for self-assembled monolayers (SAMs) with single-molecule break junction and multi-molecule EGaIn data from other groups. For the benchmark OPE-series, we consistently find an exponential decay of the conductance with molecular length characterized by β = 0.37 ± 0.03 Å(-1) (CP-AFM). Remarkably, for the second series, we do not only find that the linearly conjugated anthracene-containing wire is the most conductive, but also that the cross-conjugated anthraquinone-containing wire is less conductive than the broken-conjugated derivative. We attribute the low conductance values for the cross-conjugated species to quantum interference effects. Moreover, by theoretical modeling, we show that destructive quantum interference is a robust feature for cross-conjugated structures and that the energy at which complete destructive interference occurs can be tuned by the choice of side group. The latter provides an outlook for future devices in this fascinating field connecting chemistry and physics.

2013

Making a match for Valinomycin: steroidal scaffolds in the design of electroneutral, electrogenic anion carriers.

The natural product Valinomycin is a well-known transmembrane cation carrier. Despite being uncharged, this molecule can extract potassium ions from water without counterions and ferry them through a membrane interior. Because it only transports positive ions, it is electrogenic, mediating a flow of charge across the membrane. Equivalent agents for anions would be valuable research tools and may have therapeutic applications, especially in the treatment of "channelopathies" such as cystic fibrosis. However, no such molecules have been found in nature. In this Account, we describe our research toward synthetic and rationally designed "anti-Valinomycins". As our core approach to this problem, we used the steroid nucleus, provided by cholic acid, as a scaffold for the assembly of anion receptors. By positioning H-bond donors on this framework, especially urea and thiourea groups in conformationally constrained axial positions, we created binding sites capable of exceptionally high affinities (up to 10(11) M(-1) for R4N(+)Cl(-) in chloroform). The extended hydrocarbon surface of the steroid helped to maintain compatibility with nonpolar media. When we tested these "cholapods" for chloride transport in vesicles, they provided the first evidence for electrogenic anion transport mediated by electroneutral organic carriers: in other words, they are the first authenticated anti-Valinomycins. They also proved active in live cells that we grew and assayed in an Ussing chamber. In subsequent work, we have shown that the cholapods can exhibit very high activities, with transport observed down to carrier/lipid ratios of 1:250,000. We also understand some of the effects of structure on the activity of these molecules. For example, in most cases, powerful transporters also act as powerful receptors. On the other hand, some modifications which favor binding do not promote transport. We gained functional advantages by cyclizing the cholapod architecture, which encloses the anion binding site. We could also simplify the structure without compromising function. A steroid-inspired trans-decalin framework has proved highly effective and may lead to agents with practical advantages. Changing an ester side-chain in this system revealed a surprising effect, whereby increased length and/or lipophilicity resulted in substantially raised activity. Although much remains to be discovered about these anionophores, their high activities and intrinsic tuneabilities bode well for applications. In future work, we plan to develop and exploit these molecules as tools for biophysical research and to explore the possibility of useful biological activity.

Lipophilic Balance - A New Design Principle for Transmembrane Anion Carriers

Despite extensive interest in transmembrane anion carriers (anionophores), the factors that govern activity are still only partly understood. Herein we report a study which identifies a new principle for anionophore design, that of “lipophilic balance”. A series of simple thioureas with identical molecular formulae has been prepared and assayed for chloride/nitrate transport activity in synthetic vesicles. The molecules differ only in the positioning of the phenylthiourea binding unit within an 11-carbon linear chain. They are shown to possess very similar lipophilicities and anion affinities, while a test for leaching establishes that they locate almost exclusively in the vesicle membranes. Notwithstanding their close similarities, activities across the series show >5-fold variation, peaking when the phenylthiourea group is centrally located. The results suggest that transport is favoured by a balanced array of lipophilic substituents, possibly because this arrangement facilitates transfer of the complexed anion into the apolar membrane interior.

Statistical Analysis of Single-Molecule Breaking Traces

2012

Observation of quantum interference in molecular charge transport.

As the dimensions of a conductor approach the nanoscale, quantum effects begin to dominate, and it becomes possible to control the conductance through direct manipulation of the electron wavefunction. Such control has been demonstrated in various mesoscopic devices at cryogenic temperatures, but it has proved to be difficult to exert control over the wavefunction at higher temperatures. Molecules have typical energy level spacings (∼eV) that are much larger than the thermal energy at 300 K (∼25 meV), and are therefore natural candidates for such experiments. Previously, phenomena such as giant magnetoresistance, Kondo effects and conductance switching have been observed in single molecules, and theorists have predicted that it should also be possible to observe quantum interference in molecular conductors, but until now all the evidence for such behaviour has been indirect. Here, we report the observation of destructive quantum interference in charge transport through two-terminal molecular junctions at room temperature. We studied five different rigid π-conjugated molecular wires, all of which form self-assembled monolayers on a gold surface, and find that the degree of interference can be controlled by simple chemical modifications of the molecular wire.

Correlations between molecular structure and single-junction conductance: a case study with oligo(phenylene-ethynylene)-type wires.

The charge transport characteristics of 11 tailor-made dithiol-terminated oligo(phenylene-ethynylene) (OPE)-type molecules attached to two gold electrodes were studied at a solid/liquid interface in a combined approach using an STM break junction (STM-BJ) and a mechanically controlled break junction (MCBJ) setup. We designed and characterized 11 structurally distinct dithiol-terminated OPE-type molecules with varied length and HOMO/LUMO energy. Increase of the molecular length and/or of the HOMO-LUMO gap leads to a decrease of the single-junction conductance of the linearly conjugate acenes. The experimental data and simulations suggest a nonresonant tunneling mechanism involving hole transport through the molecular HOMO, with a decay constant β = 3.4 ± 0.1 nm(-1) and a contact resistance R(c) = 40 kΩ per Au-S bond. The introduction of a cross-conjugated anthraquinone or a dihydroanthracene central unit results in lower conductance values, which are attributed to a destructive quantum interference phenomenon for the former and a broken π-conjugation for the latter. The statistical analysis of conductance-distance and current-voltage traces revealed details of evolution and breaking of molecular junctions. In particular, we explored the effect of stretching rate and junction stability. We compare our experimental results with DFT calculations using the ab initio code SMEAGOL and discuss how the structure of the molecular wires affects the conductance values.

2011

Evidence for quantum interference in SAMs of arylethynylene thiolates in tunneling junctions with eutectic Ga-In (EGaIn) top-contacts.

This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga(2)O(3) as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10(-1)A/cm(2) at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 <

Formation of high-quality self-assembled monolayers of conjugated dithiols on gold: base matters.

This Article reports a systematic study on the formation of self-assembled monolayers (SAMs) of conjugated molecules for molecular electronic (ME) devices. We monitored the deprotection reaction of acetyl protected dithiols of oligophenylene ethynylenes (OPEs) in solution using two different bases and studied the quality of the resulting SAMs on gold. We found that the optimal conditions to reproducibly form dense, high-quality monolayers are 9-15% triethylamine (Et(3)N) in THF. The deprotection base tetrabutylammonium hydroxide (Bu(4)NOH) leads to less dense SAMs and the incorporation of Bu(4)N into the monolayer. Furthermore, our results show the importance of the equilibrium concentrations of (di)thiolate in solution on the quality of the SAM. To demonstrate the relevance of these results for molecular electronics applications, large-area molecular junctions were fabricated using no base, Et(3)N, and Bu(4)NOH. The magnitude of the current-densities in these devices is highly dependent on the base. A value of β=0.15 Å(-1) for the exponential decay of the current-density of OPEs of varying length formed using Et(3)N was obtained.

Controlling the interparticle distance in a 2D molecule-nanoparticle network

Mechanically controllable break junctions allow for an impressive level of control over the distance between two electrodes, but lack stability at room temperature. On the other hand, two-dimensional (2D) networks of nanoparticles bridged by molecules form a stable device structure for investigating molecular conductance properties. Here, we combine both techniques to create a robust platform for molecular charge transport with control over the inter-electrode distance on the picometer scale. The resistance change due to bending of our structures is dependent on the molecular species present between the nanoparticles.

An MCBJ case study: The influence of pi-conjugation on the single-molecule conductance at a solid/liquid interface.

π-Conjugation plays an important role in charge transport through single molecular junctions. We describe in this paper the construction of a mechanically controlled break-junction setup (MCBJ) equipped with a highly sensitive log I-V converter in order to measure ultralow conductances of molecular rods trapped between two gold leads. The current resolution of the setup reaches down to 10 fA. We report single-molecule conductance measurements of an anthracene-based linearly conjugated molecule (AC), of an anthraquinone-based cross-conjugated molecule (AQ), and of a dihydroanthracene-based molecule (AH) with a broken conjugation. The quantitative analysis of complementary current-distance and current-voltage measurements revealed details of the influence of π-conjugation on the single-molecule conductance.

2010

Using bis(pinacolato)diboron to improve the quality of regioregular conjugated co-polymers

We demonstrate the use of bis(pinacolato)diboron to directly polymerize symmetric, bisbromo, thiophene-based monomers via a Suzuki homo-polymerization to form co-polymers in less steps than the corresponding co-polymerization. We compare this method to the commonly used Stille co-polymerization by preparing four thiophene-based co-polymers using both methods. We use MALDI-TOF mass spectrometry to show that this new method produces high-quality, uniform polymers with narrow distributions of end-groups. By varying the electronegativity of the monomers, we demonstrate rudimentary control over these end-groups, forming either bis-H-, mono-H-mono-Br-, or bis-Br-terminated polymers in order of increasing electronegativity.

2006

Synthesis and properties of an anthraquinone-based redox switch for molecular electronics

The synthesis of a molecular wire bearing an anthraquinone core and thioacetyl end groups for gold electrode binding is described. A model anthraquinone system, substituted with tert-butylthio groups, can be reversibly switched electrochemically from cross conjugated (low conductance “off”) to linear conjugated (high conductance “on”) via two-electron reduction/oxidation reactions. This feature holds promise for the anthraquinone-based wires to be used as redox-controlled switches in molecular electronic devices.